I think there are 30 rows x 30 columns = potentially 900 cars, in reality somewhere between 870 and 900.
Pretty sure you mis-counted. Please confirm as follows.
At 0:00, there are 29 rows on screen (and 28 columns).
Cars to watch at 0:00... 2nd column, bottom 3 cars = red, yellow, purple.
Last column (right) at the top - the 3 cars = white(ish), dark blue, white (ish)
So at 0:01 (end of), user scrolls up and left slightly.
Watch last column but 1... Top 4 cars = red, white, dark blue, white.
So that's 1 extra row.
... and again at 0:03, user scrolls down.
Watch 1st column (was 2nd)...
The bottom 4 cars are red, yellow, purple, purple.
So that's another extra row.
Hence 1 row up + 1 row down plus the 29 rows on screen at the start = 31 rows. We only see one extra column scrolling right, hence 29. 31 x 29 = 899.