That seems a little odd to me. When I went through my own IVF (before my DE IVF), they measured every single little follicle and measured each of them daily. They did guess that those under, I believe it was 10, were unlikely to be mature enough to produce an egg at retrieval.
With my donor cycle, my donor had at least 45+ follicles, 27 of which had eggs, 24 of which actually fertilized on day 1.
Maybe they are just giving you the estimate of which follicles they believe are mature enough to fertilize??