Without looking at the actual traffic at the time the gap could be there for a variety of reasons - the tower might want a gap to get a departure away, there might be a medevac chopper into EN that needs the gap, the traffic disposition might have been just right to allow some track shortening, perhaps the controller was being cautious and thought the front aircraft would be slow when it wasn't.
Unless it's a sight and follow approach needs to keep wake-turbulence separation as well as radar separation - for a pair of heavies that's 4NM and for a pair of mediums it's 3NM. Of course time is also needed to vacate the runway - high speed exits versus full length.
A 15 mile gap isn't particularly excessive if you're a medium following an international heavy - anything less than 12 miles (with matching ground speeds) when I hand you off at ARBEY for RWY34 or 27 and approach will have to slow you further to keep 5 miles wake turbulence on final. A medium following a super needs 7 miles, plus whatever for the super to vacate.
It's not an exact science - different aircraft types fly differently (Airbuses are generally slower than Boeings for similar sized aircraft) and different airlines fly differently. To an extent we're trying to predict (guessing) how each aircraft will be performing however many minutes into the future and planning the sequencing accordingly, along with accommodating random events intruding.
Mixing types and categories does increase the workload mainly due to different performance characteristics - a domestic 737 might be 10,000ft higher than an international A330 on profile at 100 miles, and still several 1000ft higher at ARBEY.