When investors require an option-pool top-up as part of a financing, the pool expansion is included in the pre-money valuation. This means the dilution from the new pool falls entirely on existing shareholders, primarily the founders, and not on the incoming investors.
The shuffle is mechanically equivalent to lowering the pre-money valuation. A $50M pre-money with a 10% pool top-up dilutes founders the same as a lower pre-money without the top-up. Sophisticated founders negotiate pool size as carefully as they negotiate price.