Figure from Grundy and Sutton 1989. Note that the dark (slow) growers are joined in their second year by a new cohort (in grey).Grundy and Sutton's experiments showed that females only reproduced with they reach large size and they have one or two broods (rarely three) with an interbrood period of 5.4 weeks. This and the positive relationship between temperature and growth rate means that most individuals born in the first brood are able to reach a large size before winter sets in and start growing faster when the breeding season arrives. Most individuals born in the second brood (a bit more than a month later) miss out on the benign growing conditions and are too small after the winter and when the next breeding season arrives they are still too small and waiting until the following breeding period makes sense to maximize their chances of reproducing successfully.
Grundy, A. & Sutton, S. (1989). Year class splitting in the woodlouse Philoscia muscorum explained through studies of growth and survivorship Ecography, 12 (2), 112-119 DOI: 10.1111/j.1600-0587.1989.tb00829.x