I was using Whattomine with 25 Th for input and comparing to my Slush dashboard. Now the numbers match up. I think it just needed time to build the 24hr average.
On a big pool like slush your earnings will match the estimate quite closely. On smaller pools there might be more variance. Don't expect to mine the same exact amount of BTC every day - some days it may be higher, some days it may be lower, but it will average about the same.