This separating of banks for accrual, but can spend either is a good thing. I've been bitten by the old way before. Now, you earn what's originally on your pairing, block out, or as flown.
Example: front DH of $800 to ANC, back DH from NRT for $2500--that's how your pairing was originally and how you blocked out. Original bank total $3300. While you are on trip, you get revised to add a SIN-NRT DH for $4000, and operate NRT-ANC, and DH from ANC for $800. That's how you ended the trip. Total bank $5600.
How it can bite you. If you spent $1000 getting to ANC and were planning on spending $2000 getting back from NRT, you were planning on being $300 under bank. But with the revision, you already spent $1000 positioning, you took scheduled mid trip for $4000, and have now spent $5000 of your $5600 bank and you can't get home on the $600 left from ANC and must spend $200 out of pocket.
Rare event you think, but I've had this happen to me at least 4 or 5 times over the past 6-7 years. Getting a mid trip DH added can kill your total bank.
This change is good and solves this problem.