Assuming the $1000 and $2500 PIA benefits listed above -- If B had waited until FRA to get half of A's benefits, it would be calculated as: $1000 of B's benefit, plus (1/2*$2500) - $1000 = $250 from A's benefit, which adds up to $1250, precisely half of A's benefit. But that only works if B waits until FRA.
If B takes the benefit at age 62 and gets only $750, then her spousal addition (when spouse files) remains at $250. So, in total, she'll get $750 + $250 = $1000.
So B gets a bump to $1000 when spouse starts collecting. Another way of looking at it is that the $250 reduction for filing early is permanent (until spouse dies, as VacationForever noted).
Bob
BTW, a lot of the confusion is due to the way the general public refers to "spousal benefit", as opposed to the official definition in law and implemented by the Social Security Administration. In the example above, the spousal benefit is $250. Not $1250. You always get your benefit first, and then the spousal benefit if it's over $0. That's why filing early permanently lowers your total payment.
This, by the way, is how the SSA and their reps use the term. When talking to them, you can get very confused if you hear "spousal benefit" and assume, incorrectly, that they're referring to the way the public uses the term.