Building logic for submission ratios

I am trying to build some logic to split form submissions on a predefined ratio, in the case below a 9:4 ratio. When I test these submissions, I get the desired 9:4 ratio, but in a live environment I am getting different ratios (most recently a 9:3 followed by a 9:4 then an 8:4).

In an older incarnation, the counts were affected by having two submissions at the exact same time, but I believe I have fixed this by using the cflock. I cannot see where the discrepancies could be coming from.

Any ideas on another way to achieve this, or any way to improve on my current code?

Thanks in advance,
Paul