Unless my math is wrong your hash rate as a percent of the pool's is 0.0000345%

And your shares (at 32 shares per shift) as a percent of the pool's is 0.0000352%

So at 32 shares per shift with 0.5GH/s, your getting right about what you should.

Regarding it flipflopping between 32 and 64 shares per shift, look at how small of a percent your hashing (or shares) are to the rest of the pools and it's easy to see why it's hard to calculate that exactly.

I have a couple GH/s at CEX,io and my results jump around a little also, but by the math I'm never getting short changed and that's what counts.