hey guys, i just got my intonation set perfect on my guitar, according to my Korg Tuner, then later today i was playing on my amp and i put it in tuner mode to check that the intonation was still correct and make sure nothing slipped, and when i fret on the 12th fret everything shows sharp on the built in tuner, but then when i go back into the korg tuner is shows everything is perfect. how come one reads sharp and one reads perfect?

Ibanez RGT42DX
Boss MT-2
because your spider has a **** tuner, that's why.
ESP Eclipse 300
Jackson RR3
Bugera 6262 head
Trace Elliot straight cab
Boss NS-2
Boss DD-3
Also, pushing the string down increases the tension slightly. The note at the twelfth fret will be slightly sharp compared to the harmonic.