Counting strikes me as a little weird, though I guess going by reverse dick size order is kinder than the opposite. My guess would be that the positions are the location seen (from the outside) to be reached when objects of the varying lengths are inserted. The reason they're not evenly spaced is simply because the vagina is not a static cavity. The tissues can inflate, flex, relax, and stretch. It looks as though she relaxed enough during the measuring process that the 4, 5, and 6 ones were all accommodated fully, but that 7 got to cervical penetration.
I... think the numbers are just the amount of attempts from her lovemaking with the... teacher was it? The lines imply the (depth) result of each lovemaking.
1st one was painful and only reached a bit in. The 2nd one went a bit better, but she was probably a bit sore still and also possibly too tight.
3rd one went a lot better, but her body was still adjusting.
4-6th ones were at the entrance of the womb.
And with the 7th, if you follow hentai logic that it is seemingly pretty easy to get into the uterus, would be the teacher's dong reaching full depth.
This probably should clarify things. Even if it is still mainly just a hypothesis from me.