How does MS shard work on Singed?

When I start a game without it, I get 1060 ms ( I make sure to ping it every game in order to mog my team) with 4 passive stacks + fountain.

When I want to do velocitymaxxing, I get 1064.

that is four more, or 0.38%, not 2.5%.

Even if the shard was only boosting the base movement speed, 0.025*345=8.625.

I tried to look into the wiki for an explanation and couldn’t find one. It seems weird to trade 9 whole ap for 1% flat ms.