Whats your hashrate?
This looks like a single usb block erupter.

Curious where you get better rewards?
No, it's just oldstyle GPUs

about 2.5Gh/s
It's not about better rewards it's about how to interpret such a huge diff in rewards for shares.
Actually, 47 shares in 7.5 minutes seems way to low for a 2.5 GH/s miner.
Ah, wait, your difficulty is set higher than 1, is it?
Later shares get higher scores, did you possibly have a little downtime near the end of that 4:20 round?