Advance is actually intended to cover charges after you terminate your account.
As for the difference is amount, it's likely they prorated your first bill.
I think your billing is correct. Your cut-off, according to Globe, is every 17th of the month. Since you had the plan on Sept. 19, there's a 2-day credit from your advance MSF of P1799 paid on Sept 19.
Sept 19 - Advance MSF - P1799 (supposedly applied from 9/19 to 10/17)
iPhone cashout - P7200
TOTAL (you paid on 9/19) P8,999
First billing: Advance MSF (10/18 - 11/17) P1799.00
Credit from initial payment -119.93
Bill # 1 due on the Nov. 7 P1679.07
Due date is usually 20 days after cut-off. Since your cut-off according to Globe is every 17th, 20 days after is the 7th of Nov for this billing period.