The scoring card gives more points when there are less players. For instance a maximum of 30 with 2/3 players versus 27 with 4/5 players.
However, the more players there are, the harder it is to gain influence. This is because the influence tracks are more crowded and, since there can be only one cube per level, it becomes harder to progress since you need bigger increments. In particular, the related patrons such as Caesar get less useful.
Is this a mistake in the card?
If not, what is the rationale behind this difference of scoring?
The card is correct. The reason for that, more player means higher influence with the brown cubes (easyer to get it on higher maximum) as there are more card movements.