The environment for CSS isn't nearly as "standard" as you might think. Even though WebKit and Blink are the majority engines on mobile there's tons of combinations of versions, screen sizes, and aspect ratios. Trivial layouts are easy but more complicated layouts can be very difficult to get working right, let alone looking right, across browsers/platforms.

The problems are often solvable but not necessarily easy. The difficulty is compounded by the heterogeneity of the Android space.

It's great the majority browser engines follow the CSS standards but a lot of problems aren't bounded by the accuracy of the implementation.