It makes perfect sense, honestly.
The big seasonal event sales generate huge buzz in games media and social media. They get larger crowds, a majority of which probably don't track Steam or digital PC game deals much or at all on any regular basis. For them the winter/spring/summer/fall sales are an opportunity to spend their hard earned cash on some good games that have at least modest discounts.
On the other hand, it's likely for games to receive their biggest discounts during random daily/weekly/weekend deals because in between the big events there isn't going to be as much traffic on Steam or other store fronts. A deep sale is the best way to advertise a product (and possibly devalue it).
It's the same thing with outlet malls, for example. People go to them during Memorial Day, Black Friday, etc. hoping to grab the best deals on clothes, kitchenware, etc. 25% off outlet prices seems like a good enough deal. However, the best deals are during the slowest times - random weekends in September or March, for example. I've done this many times and have found great deals, especially clearance plus additional 30-50% off that makes the overall discount >75%, on things I needed.
tl;dr - bigger crowds and more advertising = less incentive to offer higher discounts
smaller crowds and slower sales periods = more incentive to offer higher discounts to get people "in the door"
(Apologies if I seem pedantic)