That kind of variation is just too complex to consider for a home. I’m surprised they even have that table for Warm Season Grass, when there is more variation between one type of warm weather grass and other (my Centipede versus Bermuda, for example), than the monthly variation shows. Such variation can be critical in farming, where hundreds or thousands of acres can result in huge amounts of money being wasted, but even looking at this non-specific table, there are some potential problems: Like April, May and July Kc all higher than June Kc. Variations like this can and do occur, due to stages of growth, like seed production, dormancy, etc. But in the end, for residential lawns, a single value is sufficient. Heck, it’s hard enough to even get specific and accurate average values for specific grass and other crops.
In the end, I think if you take actual or average values for ET over these months with a constant Kc, or with varying values of Kc, the results will be close and the complexity it would cause is really just not worth it. Most people have much higher errors in their other values for this to matter much in comparison.