have you noticed holes/trench on the runways?

[II./JG1_Klaiber]

Since the airfields you're talking about are probably Class A (Class A meaning airfields constructed to the specifications set by the British Air Ministry in August 1942, whose main feature was a set of converging strips in a triangle pattern), it's extremely possible that what you're seeing is actually some form of "tile drainage" system.

Tile drainage is the practice of removing excess water from the subsurface of soil intended for other uses.

If the real airfields that the in game versions are based of off were lengthed at any point through swampy land, like RAF Bassingbourn was, this may be what's going on.

But this is just a complete guess...