Wow! I didn't think biro would come of quite that easily but I knew it fades over time.
This is the reason documents that people want to last are never written in biro. Documents like birth certificates, marriage certificates, deeds for grave plots etc.
Wet ink, fountain pen, sort of soaks into paper in a way biro or photocopy ink doesn't.
If you are trying to determine if the copy of a signature is original see if you can smudge it with a wet finger, If its real it will smudge if its a photocopy it won't.
I've just googled the difference fountain pen is water based, ball pen is oil based.