the calculations might need some adjustment …

my home matches:
league match against bottom of table: sold out (53.000)
CL return match (2×3 away): not sold out (49.391)

I’d rather understand when league matches don’t attract too many people …

We can adjust it but the reason that happened considering the current calculations is that you’re in the top of your league table so even though you played against one of the teams in the bottom your supporters will be there due to your good position. In the CL match it’s still the first round so the attendance is not as high as it will be in the following rounds.