You may mock, but I'm just checking. Here is the reason (using my genuine figures)

Current Balance: 144,000
Due to expire end Nov: 5,800

So come 01 DEC, my balance should be 138,200.

HOWEVER

I'm going to spend 4,800 miles (4 x 1,200) to extend a bunch of other miles. So things could do one of two ways:

Worst Case Scenario
Current Balance: 144,000
Miles used to extend other miles: 4,800
NOV balance: 139,200
Due to expire end Nov: 5,800
DEC balance: 133,400

Best Case Scenario (expected)
Current Balance: 144,000
Miles used to extend other miles: 4,800
NOV balance: 139,200
Due to expire end Nov: 5,800 - 4,800 used = 1,000
DEC balance: 138,200

See what I mean?