The more slipways a shipyard has, the faster it takes to retool the shipyard, even though the BP cost increases.
For example, if it costs 171 to retool a 1-slipway shipyard with 1680 mod rate on 25th Oct, it completes on the 1st Dec.
If I change the number of slipways to 2, it costs 228.1 as expected, but completes on 19th Nov.
If I change the number of slipways to 4, it costs 342.1 as expected, but completes on 13th Nov.
It doesn't just predict these dates, it actually retools this fast. I suspect the # of slipways is being divided out to simulate parallelism, but parallelism is already built into the build cost.