So what's up here? Is it my brain or mIRC which seems to be unable to calculate?

$calc(3893208833356031+6864822661557624) - This isn't the hardest thing to calculate yet mIRC seems unable to find a proper answer.

It should return '10758031494913655' yet it returned '10758031494913656'.
What's going on guise?

ps if you change the "...624" into "...625" You should notice the bug.