Overnight builds are specified by hours of wear rather than by absorbency alone: a higher core load helps, but waist-to-back coverage and the standing leg cuff are what prevent leaks while lying down. Compared with a day-weight product the chassis runs longer and the back panel taller. Ask which absorbency standard the figures come from, because ml ratings measured under different test methods are not comparable. Compare the test method before you compare the numbers:
how absorbency is measured.