Semi-Circular rule explain
 

Hello!
I would like to ask about the semi-circular rule for flight levels as I thought I understand it pretty well, but then looked at the flights like for ex.: EGSS-LPPT or LPPT-LIME. Compared to the rule, these flights have their cruising altitude as a total opposite. (EGSS-LPPT) as it is flying southwest with a heading in the range of 180-359 deg. it should be cruising in an even flight level. Obviously they usually cruise in FL370. Now comes the question...why is that? Is there some kind of restriction?
Thank you very much for all clarifications and happy landings!