Hi,
Can you give more information about the images you use to check this?
I tried with random Tensors but I can’t see the same thing:
a = torch.rand(250, 250)
b = torch.rand(250, 250)

F.l1_loss(a, b, reduction='mean').item() - np.mean(np.abs(a.numpy() - b.numpy()))
> 2.9802322387695312e-08

What can happen is that the reduction is imprecise if the values in your image are large.
Can you try comparing to (a - b).abs().mean() for torch as well?