Size table is only a guide.
An Impact Vest / neoprene vest and jackets for water sports should sit tight for three main reasons: 1: Neoprene stretch, so the vest will get bigger. 2: It should hold together the body and distribute pressure at eg. falls and provide a support for the back and chest. 3: The vest should remain in place when you end up in the water and not slide up, but also not be in the way when riding. (Take therefore the smaller size if you are unsure.)
Impact vests Life jackets which provide protection, support and buoyancy to the rider, but is not CE approved life jacket that. These life jackets are usually neporene vests adapted to water sports demands of mobility, protection, etc. and then not meets all requirements as ISO marking requires and has a limited buoyancy. Or which may also be the case that they simply have not been tested since this is a complicated and costly classification that also usually means that the vests are not fully adapted to the requirements of watersports use and therefore are not always popular.