You made the mistake of using lots of low numbers (31 or lower). Those numbers tend to be chosen by people who use birthdays for their selections. So any draw where all 6 numbers drawn are under 32 (as happened tonight) is likely to have far more people with 4 numbers than normal.
There were 14,811 people with 4 numbers tonight.
Going back through the past few week's draws (with the most recent first) here's how many people had 4 numbers:
11,913 (Sat, when more people play, so there are bigger prize funds, £133 payout)
5,232 (Wed, £178)
8,658 (Sat, £211)
9,717 (Wed, £61)
17,025 (Sat, with only one number greater than 31 - proving my point, £70)
As usual, I got no numbers tonight. (I do the lottery, with one line, on both Wed & Sat. I've not had any win for well over a year).