Andrew,
This is common in the online IGI and is something one needs to look out for.
Many people (even including people who run LDS Family History Centres!) have tried to extract a meaningful answer from the LDS - but without success!
If you can get your hands on the old microfiche I understand that you will sometimes find the missing boys there. Otherwise (and, of course, for extractions which have been done since those days) it is a case of ordering in the relevant film to a local LDS FHC. Or, for Scotland, consulting the Scottish Church Records CD which is available at the FHCs and which includes the missing boys.
JAP