Possible contradiction between number of Slytherins in PoA and 'the original 40?'

Question

I was reading Harry Potter and the Prisoner of Azkaban again and noticed something peculiar.

In PoA, the book explicitly states

Three-quarters of the crowd was wearing scarlet rosettes, waving scarlet flags with the Gryffindor lion upon them, or brandishing banners with slogans like "GO GRYFFINDOR!" and "LIONS FOR THE CUP' Behind the Slytherin goal posts, however, two hundred people were wearing green;

The amount of Slytherins here do not add up to the original forty, where it is safe to assume that there were ten students in each house from the original forty:

40 students / 4 houses = 10 students/house every year

So, with around 40 students in each year, that is:

10 students/house x 7 years = 70 students/house

This is assuming that everyone stayed after O.W.L.s which isn't likely. We also have to remember to subtract the amount of people actually playing Quidditch.

70 students - 7 students = 63 students

Now, lets add ten teachers that may be on rooting for Slytherin, also unlikely.

63 students + 10 teachers = 73 Slytherin fans.

If we wanted to go even further, we can add at most 20 people who are from other houses, rooting for Slytherin. This is incredibly unlikely because of the previous statement:

Three-quarters of the crowd was wearing scarlet rosettes...

'

63 Slytherin fans + 20 Others = 83 Slytherin fans.

83 is way lower than 200, so is there something going on here or am I missing something?

Answer 1

In addition to comment about writers can't do math.

The most plausible solution is that the students from other houses also supported Slytherin. It contradicts three fourth of croud wearing the scarlet. but seeing there are only about 300 students in the castle. 200 Slytherins doesn't make sense.

The movie however helps us with an insight. Lucius Malfoy is also seeing the match between Gryffindor and Slytherin in Chamber of Secrets. There are only 12 quiditch games in a season at hogwarts. Considering the quiditch as a high school game, where parents and relatives are allowed to see the children play, we can assume the crowd not only contains the students but adult witches/wizards as well. Consider the scouts of major teams, parents, relatives, casual quiditch fans. We can easily get the number supporting Slythering upto 200.

This is also not the only time non-students were allowed in Hogwarts. Triwizard tournament had Weasleys visiting the castle before the Final Task.