8 = 8

[napalm-sandal.livejournal.com]

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This is a weird question to come here with, but crazy people tend to be smart.

[kutera.livejournal.com]

Whenever I input this number, 72057594037927935, into an online calculator, it automatically turns into 72057594037927940. When I try to subtract 5, nothing happens.
Is this some kind of glitch that occurs in all online calculators (I've tried two different ones)? Or have I found the secret bugged number or something?