There is a high degree of variability in the digestibility of chitin (in insectivores) ranging from 2 to 85 percent, so to label it as indigestible is not correct until the level of digestibility for that species is determined. Even for animals with low digestibility percentages, the undigested portion of chitin serves the same purpose as fiber in supporting gut motility.

Also, larger invertebrates will often have a lower total percentage of chitin, as there is disproportionately less chitin when compared to the total volume of the insect. Its digestibility also is influenced by the presence of certain symbiotic bacteria and protozoa, whose populations can fluctuate (with the use of antibiotics, change in stress levels ect), so even within the same species the digestibility has the potential to fluctuate over time.

Some literature...

http://ci.nii.ac.jp/naid/110003372739/en/
http://www.jhc.org/cgi/reprint/50/8/1081.pdf