As you know computers use a binary system. So in stead of 10 100 1000 they're counting like 2 4 8 16 .... 1024.
That's why the computer wizards of old decided that a kilobyte would better fit when it was 1024 bytes in stead of 1000.
Whenever kilobytes are mentioned check whether they're written kB or KB, the lower case k is the official SI prefix meaning 1000, the capital K is often used as 1024.
To confuse things even more the M for mega means either 1000*1000 or 1024*1024 and there's no way to distinguish them. (Yes the problem even gets worse for gigabytes.)
This is the source of your confusion.
So when 1000 KB doesn't equal 1 MB then you should try 1024 KB.

And to complicate things even more. When somebody's telling you he's sending a 900 KB attachment, the message will be 1200 KB, due to necessary encoding.

I thought if I waited long enough some spam would have arrived giving me an example to work with.

I just received 2 spam e-mails. On my ISP they read that they are 2k in size. Yes, a little k.

However, in The Bat! One reads 1,514 and the other 1,829.

Meanwhile, I also got a spam e-mail that my ISP reads that it is 1k in size. The Bat! reads 729b.

You can see how this would be real confusing for me. The numbers are not exact they contradict each other.