Slush says: "Estimated reward"... meaning it's an estimation only.. from what I can see based on the current hashrate of your miner(s)

You don't get the "Estimated reward" at the end of the block but in most cases something close to that.

After finishing a block, which can take over a day as we witnessed multiple times recently, you´ll see the status of the block change to processing. Only then every share you participated is calculated and the final payout determined.

I´m not sure of the exact calculation method for the estimated reward but it seems that it´s related mostly to your average hashrate. This explains why the estimation start to drop to zero after you switch pools or switch off your miners. However upon finalizing the block you´ll still get your rightful share...

BTW it´s  a know factor that payouts on Slush vary greatly, there´s simply not enough mining power (about 3 % of the total available) to sustain steady payouts.. so it all comes down to luck..

If you're looking for more steady payouts you should either opt for load balancing or entirely switch to a larger pool.