I just hanged-up with a very friendly agent at AMEX-BA customer service, but I still don't get it.

In my last statement (december 26th), I have spent a total of £2142.98 in my AMEX BAPP, of which £558.80 were with BA. I have no cash withdrawals, transfers or anything not qualifying for miles earning.
In the back of the statement (and in ba.com) says I earned 1676 miles for BA spend, which is correct (558.8 * 3 = 1676). However, my miles from non-BA spend is 1805, which seems to be incorrect ((2142.98 - 558.8) * 1.5 = 2376).

The agent said even it was a bit confusing in the statement, everything was fine and the miles are correctly posted, and I believe him, but can someone please explain to me how they do actually calculate the miles?
Cheers!