Cells change size more readily than number
Fat is stored in adipocytes, and when adults gain or lose fat the dominant change is usually in the size of existing cells rather than their count. Research suggests the total number of fat cells is largely set by adolescence and that the body turns adipocytes over slowly, so weight loss mainly shrinks cells rather than removing them.
When fat gain is very large and sustained, the body can recruit new fat cells, raising the overall cell number.
Why this can raise the long-term floor
Because adult fat-cell number tends not to fall much even after substantial weight loss, someone who once carried a great deal of fat may retain a higher count of smaller cells. Some researchers propose this contributes to the difficulty of maintaining weight loss, since more cells can refill, though this is one factor among many and not a complete explanation.
Hormones, behavior and environment also shape regain. This is general educational information, not medical advice.