How do I solve Soparla's riddle?

Consider each group of numbers? What do they each have in common with each other?

The relationship is not mathematical as such.

Or maybe it is in a roundabout way.

Look at the numbers as words.

And then count the letters.

1-2-6 all have three letters. 4 5 9 all have four. 3 7 8 all have 5.