Basically, AMD's APUs are updated every 3 months as WHQL releases only, whereas the firm's discrete graphics card often receive multiple driver updates per month. Obviously, this is not ideal for gamers as this leaves a lot of room between a game's release date and the time AMD provides performance optimizations and/or bug fixes.
This also leads to a somewhat funny situation. A couple of days ago, Intel rolled out driver version 18.6.1 for its Kaby Lake-G processor with the Radeon RX Vega M integrated graphics. Meanwhile, AMD's own APUs are still stuck with 18.5.1.