I see it as this:
5 - tighthead lock, as he packs down behind the tighthead prop in the scrum. The tighthead prop is under more pressure in the scrum, thus in order to help the tighthead prop, the tighthead lock is usually the stronger scrummager of the two locks. This usually translates into him being the heavier of the two.
4 - loosehead lock, picked more for his lineout abilities, with less emphasis on scrummaging. Likely to be the more mobile, thus will have slightly different roles in attack and defence.
That's the way I see it anyway.
See, similar reasoning could justify playing them at 4. You can support your tighthead at 5, or add pressure on the loosehead side from 4. Perhaps it's more of a reason to play at 4. Complete novice at scrummaging, but I always took the tighthead to "lock" the scrum and the loosehead to disrupt. In which case, perhaps having that extra pressure at 4 helps. Come to think of it, that might be why Charteris and AWJ switched sides. Back when you had the best tighthead in the world, you might as well throw the scrummaging lock (AWJ) behind Gethin. But since Jones stopped being picked, AWJ has been needed more at 5 to help the younger tightheads.I see it as this:
5 - tighthead lock, as he packs down behind the tighthead prop in the scrum. The tighthead prop is under more pressure in the scrum, thus in order to help the tighthead prop, the tighthead lock is usually the stronger scrummager of the two locks. This usually translates into him being the heavier of the two.
4 - loosehead lock, picked more for his lineout abilities, with less emphasis on scrummaging. Likely to be the more mobile, thus will have slightly different roles in attack and defence.
That's the way I see it anyway.