Many modern cpu waterblocks are bowed by design. They straighten up when mounted on CPUs with cpu mounting sys bringing lot of mounting pressure, and in process of it by such design bring out better thermal contact in middle. There been cases when user had lapped their block perfectly flat when unmounted with getting worse cooling as result (when mounted such block presses less at center where behind IHS cpu chip is, or even you get some concave in middle, with even worse thernal contact then with lacking pressure. IIRC this bad lapping case was with watercool HK3 blocks, but probably can be attributed to most modern blocks).