Yes, that really is bizarre... but so, too, is the effect created by capping the PAS pay rate by trade. While what Leon J points out about being better off as a PAS Flt Lt than a Wg Cdr CS is correct for pilots, navs and other WSO types are prevented from reaching the top of the PAS pay scale (strange because, when the scheme was brought in, we were actually shorter of navs than we were of pilots). The upshot of this is that a nav who thinks he's got a decent crack at becoming a Wg Cdr has the incentive to stay on CS and get promoted, and if he's successful will get a better pension and be paid more during his service career than if he chose PAS.
Could this be why we seem to have a surprisingly large number of Wg Cdr navs while our front-line fleets are going more and more to aircraft that don't employ navigators, either because they don't have a crew position (Typhoon, C-130J, C-17 etc) or because their services are no longer required (SH)?