Because it involves two very well-known teams, unlike other games that involve well-known teams against little-known teams. For the Rangers game against Panathinaikos, it will be a difficult game to predict, both teams finished second in their leagues last season and I don't see much difference between the two
Both teams are well-known & considered strong in their respective leagues, but the difference in strength is clear in this match. The quality of the Scottish Premiership is superior to the league where Panathinaikos & Olympiacos play. For that reason, we can also see different odds in the Rangers - Panathinaikos match. So far, for 1X2 bets, I see Rangers getting @1,86 while Panathinaikos gets @4,00. I've no plans to bet on this match, but if I had to, I think I would choose Rangers over Panathianaikos.