All that is going on is that numpy is summing across the first (0th) and only axis. Consider the following:

In [2]: a = np.array([1, 2, 3])

In [3]: a.shape
Out[3]: (3,)

In [4]: len(a.shape) # number of dimensions
Out[4]: 1

In [5]: a1 = a.reshape(3,1)

In [6]: a2 = a.reshape(1,3)

In [7]: a1
Out[7]: 
array([[1],
       [2],
       [3]])

In [8]: a2
Out[8]: array([[1, 2, 3]])

In [9]: a1.sum(axis=1)
Out[9]: array([1, 2, 3])

In [10]: a1.sum(axis=0)
Out[10]: array([6])

In [11]: a2.sum(axis=1)
Out[11]: array([6])

In [12]: a2.sum(axis=0)
Out[12]: array([1, 2, 3])

So, to be more explicit:

In [15]: a1.shape
Out[15]: (3, 1)

a1 is 2-dimensional, the "long" axis being the first.

In [16]: a1[:,0] # give me everything in the first axis, and the first part of the second
Out[16]: array([1, 2, 3])

Now, sum along the first axis:

In [17]: a1.sum(axis=0)
Out[17]: array([6])

Now, consider a less trivial two-dimensional case:

In [20]: b = np.array([[1,2,3],[4,5,6]])

In [21]: b
Out[21]: 
array([[1, 2, 3],
       [4, 5, 6]])

In [22]: b.shape
Out[22]: (2, 3)

The first axis is the "rows". Sum along the rows:

In [23]: b.sum(axis=0)
Out[23]: array([5, 7, 9])

The second axis are the "columns". Sum along the columns:

In [24]: b.sum(axis=1)
Out[24]: array([ 6, 15])

I use a nested loop operation to explain it.

import numpy as np

n = np.array(
[[[1, 2, 3],
 [4, 5, 6],
 [7, 8, 9]],

 [[2, 4, 6],
 [8, 10, 12],
 [14, 16, 18]],

 [[1, 3, 5],
 [7, 9, 11],
 [13, 15, 17]]])

print(n)

print("============ sum axis=None=============")

sum = 0
for i in range(3):
  for j in range(3): 
    for k in range(3):
      sum += n[k][i][j]
print(sum) # 216

print('------------------')
print(np.sum(n))  # 216
print("============ sum axis=0 =============") 
for i in range(3):
  for j in range(3):
    sum = 0
    for axis in range(3):
      sum += n[axis][i][j]
    print(sum,end=' ')
  print()

print('------------------')
print("sum[0][0] = %d" % (n[0][0][0] + n[1][0][0] + n[2][0][0]))
print("sum[1][1] = %d" % (n[0][1][1] + n[1][1][1] + n[2][1][1]))
print("sum[2][2] = %d" % (n[0][2][2] + n[1][2][2] + n[2][2][2]))
print('------------------')
print(np.sum(n, axis=0)) 
print("============ sum axis=1 =============") 
for i in range(3):
  for j in range(3):
    sum = 0
    for axis in range(3):
      sum += n[i][axis][j]
    print(sum,end=' ')
  print()
print('------------------')
print("sum[0][0] = %d" % (n[0][0][0] + n[0][1][0] + n[0][2][0]))
print("sum[1][1] = %d" % (n[1][0][1] + n[1][1][1] + n[1][2][1]))
print("sum[2][2] = %d" % (n[2][0][2] + n[2][1][2] + n[2][2][2]))
print('------------------')
print(np.sum(n, axis=1))  
print("============ sum axis=2 =============") 
for i in range(3):
  for j in range(3):
    sum = 0
    for axis in range(3):
      sum += n[i][j][axis]
    print(sum,end=' ')
  print()
print('------------------')
print("sum[0][0] = %d" % (n[0][0][0] + n[0][0][1] + n[0][0][2]))
print("sum[1][1] = %d" % (n[1][1][0] + n[1][1][1] + n[1][1][2]))
print("sum[2][2] = %d" % (n[2][2][0] + n[2][2][1] + n[2][2][2]))
print('------------------')
print(np.sum(n, axis=2))
print("============ sum axis=(0,1)) =============") 
for i in range(3):
  sum = 0
  for axis1 in range(3):   
    for axis2 in range(3):
      sum += n[axis1][axis2][i]
  print(sum,end=' ')

print()
print('------------------')
print("sum[1] = %d" % (n[0][0][1] + n[0][1][1] + n[0][2][1] +
              n[1][0][1] + n[1][1][1] + n[1][2][1] +
              n[2][0][1] + n[2][1][1] + n[2][2][1] ))
print('------------------')
print(np.sum(n, axis=(0,1)))

result：

[[[ 1  2  3]
  [ 4  5  6]
  [ 7  8  9]]

 [[ 2  4  6]
  [ 8 10 12]
  [14 16 18]]

 [[ 1  3  5]
  [ 7  9 11]
  [13 15 17]]]
============ sum axis=None=============
216
------------------
216
============ sum axis=0 =============
4 9 14 
19 24 29 
34 39 44 
------------------
sum[0][0] = 4
sum[1][1] = 24
sum[2][2] = 44
------------------
[[ 4  9 14]
 [19 24 29]
 [34 39 44]]
============ sum axis=1 =============
12 15 18 
24 30 36 
21 27 33 
------------------
sum[0][0] = 12
sum[1][1] = 30
sum[2][2] = 33
------------------
[[12 15 18]
 [24 30 36]
 [21 27 33]]
============ sum axis=2 =============
6 15 24 
12 30 48 
9 27 45 
------------------
sum[0][0] = 6
sum[1][1] = 30
sum[2][2] = 45
------------------
[[ 6 15 24]
 [12 30 48]
 [ 9 27 45]]
============ sum axis=(0,1)) =============
57 72 87 
------------------
sum[1] = 72
------------------
[57 72 87]