Q.Why Is My Lily Of The Nile Blooming In Late October?
This is the second year for the plant. It was blooming in the summer when I purchased it. I did try to report it. The root system is very strong. It didn’t bloom this summer, however, I put it in the potting shed at the beginning of October because it was getting cold during the nights in my area. After a few weeks I noticed that it has two blooms on it. I have brought it into the house to enjoy it as it flowers.

Certified GKH Gardening Expert
It depends on the variety as to when it blooms. Some don't bloom till fall, especially the white ones. The nursery may have forced the blooms early to sell them before fall.
https://www.gardeningknowhow.com/ornamental/bulbs/agapanthus/agapanthus-flowering.htm