Sid - this could simply be because their service lives (per type) could (and did) see many changes of use during their lifetime - Halifaxes and Stirlings for example becoming glider tugs and transports, and in the case of the Halifax a successful career in Coastal once they became available, with only late marks of Halifaxes remaining in "combat" service to the end of the war. The last "combat" sortie by a Stirling was by an aircraft of 149 Sqn on the 8th of September 1944...but the type had a very successful tow/transport career on "retirement"! It's also possible that their various "later" duties/careers meant lowered airframe and engines stresses etc...If a picture is emerging on the limited evidence here available, it seems to be that multi-engined types achieved more hours than single engined types.
On the other hand - aircraft like Spitfires had only limited re-use possiblities once particular marks became combat-obsolete; ASR (dinghy dropping), "Jim Crow" flights, OTUs, Coastal Artillery Co-op etc. was only going to soak up a small percentage of Spitfire marks as they successively became obsolete...
On the issue of stresses - I've a feeling that a single-engined fighter type would ALSO spend a greater percentage of its lifetime hours on "full war power" with associated fast engine wear, than a bomber on best economy cruise to distant targets. In turn - in comes the factor that it might simply be seen as more longterm-cost effective to re-engine a multi-engined aircraft than a single-engined combat type...which would soon be obsolete anyway I mean...a multi-engined type can survive to go on to other useful uses and accumulate a high number of flying hours...but with the supply certainly outstripping the demand for alternative uses for aged/obsolete single-engined fighter types...why bother teasing out their lifetimes???