The difference is KHMX transmits with roughly 4X as much HD power as KRBE.
KRBE-HD broadcasts at the original -20 dBc power level. KHMX-HD broadcasts at the more recently authorized -14 dBc.
In other words, KRBE's IBOC ERP is down 20 dB from their analog ERP. KHMX's IBOC ERP is down 14 dB from their analog ERP.
In even simpler terms, for a 100kW ERP, a -20 dBc station has a digital ERP of 1kW. A -14 dBc station has a digital ERP of 3.98kW.
Is anyone is Houston -10dBc