Very generally speaking I would suggest that the chances are most pilots flying twins would be flying IFR, closer to what you would do with an airline. The aircraft are more complex and require more skills to qualify to fly, again, one step closer to airline flying. They have two engines which require another skillset to fly, again closer to an airline operation. They fly faster (generally) than the average single and therefore necessitate a greater mental capacity to fly, one step closer to the airline operation.
So, it's only natural, and realistic to put greater value on these flying hours as far as an airline employer is concerned. How much value to put on them is a mute point. If someone has 700hrs single the majority of which was built flying A to A VFR and another pilot has 700hrs of which 300 is twin IFR A to B I would suggest that his 300 twin are worth a lot more than double his single hours. If airlines view it the same it's because experience has shown them that the more experienced (in terms of quality) represents less of a training 'risk' (read 'cost').
PP
edited to agree with Gorky's comments re-turbine singles.