If i already paid $6300+ why does iRS now say i owe an additional $315?

[Rainman12]

Late penalties can't be calculated and billed until the original amount owed is paid.

[VolvoGirl]

That's common for the IRS to send you a bill for more Penalty.  It's based on when they actually get your tax due payment.  Turbo Tax only can calculate an estimated penalty.