You have modded transport belts to have a speed of 16 items per second - which would be fantastic for the other ratios involved in this mod - however due to the internal precision of items on belts (1/256th of a tile at a time), belt speeds can only be multiples of 1.875 items per second, and will round down to accomplish that. This means that despite advertising 16 items per second, your belts still only move 15 items per second. I tested this with a belt throughput measuring circuit and found that it did indeed only carry 900 items per minute, ie. 15 per second.

Unfortunately there isn't really a "good" fix to this. You can set the belt speed to 9/256 (up from vanilla belt's 8/256) but this would give a speed of 16.875 items per second - and the tooltip would show this ugly decimal - so you can still carry the 16 items per second (with some extra leeway).

More info: https://lua-api.factorio.com/latest/prototypes/TransportBeltConnectablePrototype.html#speed

That's unfortunate, I wanted to go 16/32/64/128 belt speeds to keep in the computer-themed nature of the mod, but if this is the case, I'll go 15/30/60/120

Thanks for informing me. Fixed for 0.2.5

It you want to keep some of the theme of powers of 2, you could note in the belt's description that it moves items 1/32 of a tile per tick (1/16 for future red belts, and so on) and have it "just so happen" to be 15 items/second when all is said and done :D

That's true and clever, but I feel that's too unnecessarily vague for the player. I think player experience is just more important in this case.