That's due to implementation -- atime may be modified at the time when the underlying fopen() or fclose() is called. I'm not sure which is the case, but either:
i. mIRC only needs to call fopen() once to read multiple times, so multiple reads won't result in multiple fopen()'s.
ii. mIRC has not yet called fclose().
I'd tell you to do your freaking research, but you've already found a site which states the following:
Any viewing, copying or moving of the documents, images and other files automatically alters last accessed date stamp of the file.
Thus, contradicting yourself and backing my statement. At least you would have learnt
something. Hopefully, your lesson learnt was to do your research before you say things that may be stupid.