Presumably the galaxy gen doesn't really work for numbers less than four, so you end up with large portions of the galaxy just not having any precursors at all.
For science™ I just made 9 test games with 1, 2... 9 precursors and looked at the save games.
It is a bit different (but also different as I thought myself before).
600 star galaxy.
9 precursors:
775 precursor tags total
84-90 tags per precursor
8 precursors:
749 precursor tags total
86-100 tags per precursor
7 precursors:
695 precursor tags total
98-100 tags per precursor
6 precursors:
600 precursor tags total
100 tags per precursor
5 precursors:
500 precursor tags total
100 tags per precursor
4 precursors:
400 precursor tags total
100 tags per precursor
3 precursors:
300 precursor tags total
100 tags per precursor
2 precursors:
200 precursor tags total
100 tags per precursor
1 precursor:
100 precursor tags total
100 tags per precursor
So, basically, the tags per precursor seem to have upper limit of 100.
This causes the "empty" spaces with fewer precursors.
Will make some more tests with different galaxy sizes.
Edit:
1000 star galaxy, 9 precursors:
900 precursor tags total
100 tags per precursor
1000 star galaxy, 4 precursors:
400 precursor tags total
100 tags per precursor
1000 star galaxy, 1 precursor:
100 precursor tags total
100 tags per precursor
200 star galaxy, 9 precursors:
305 precursor tags total
30-38 tags per precursor
200 star galaxy, 2 precursors:
200 precursor tags total
100 tags per precursor
200 star galaxy, 1 precursor:
100 precursor tags total
100 tags per precursor
No matter the galaxy size, max tags per precursor is 100.
So, in a 1000 star galaxy you will have empty space even with 9 precursors, while in a 200 star galaxy you will have no holes even with only 2 precursors.
The only anomaly is 305 precursor tags in a 9 precursor 200 star galaxy, which suggests that a system can have multiple precursor tags.
Actually, looking at the game files more closely, I found a system with 3 precursor tags there. I looked at the 600 star, 9 precursors save again and around 33% of all systems with precursors have 2-3 of them. So they seem to be "clumped" a bit.