Quite likely their accuracy is not tested by the software publisher. To the extent they are tested at all, they are tested using their own servers, in-house, perhaps over a LAN.

It is very important to test code under the same configuration that it will actually be used by real users.

In the case of network I/O, quite commonly the progress bar jumps when some data has been transferred into a kernel buffer, but the actual transmission of the data takes place in the kernel, and is not directly visible to userspace code.