Your heart hasn’t learnt to cope with a higher max rate. Your body has learnt how to place a higher demand on your heart while cycling that is pushing it closer to the max that it’s able to cope with.
For context, I’ve always found my max HR when cycling to be lower than other sports I’ve done which recruit more muscles, like rowing and running. When I first started cycling this was quite a big gap, as much as 10bpm. The more cycling I’ve done, the more the gap has narrowed, which I put down to all that cycling training leading to stronger cycling muscles, better bloodflow to them (capillarization), learning how to more effectively use my upper body muscles for stabilisation when doing maximal cycling efforts, etc. All of which adds up to being able to generate a greater overall demand for oxygen when cycling, hence higher max HR.
Increased ability to suffer maybe also a factor if you didn’t have much background in pushing your body to it’s limits before you got into TR and cycling.