Why would an airport use an L/R runway combination and a second number instead of L/C/R?
There is flexibility on the numbering of parallel runways based on a number of factors as described in FAA advisory_circular/150-5340-1J -page 4 (see excerpt below). In the case of KPHX for example, there is the main terminal between runways 8/26 and 7R/L-25R/L.  Also, when approaching from the east the threshold for runway 25L is a significant distance from the threshold for runway 25R (about 2500 feet inset).
I would guess that minimizing possible pilot confusion factored into the decision not to have a &quot;center&quot; runway designation.