The Waukesha brochure indicates a gross power at 5000bhp at 100degF at up to 3000ft altitude, so no derate for temp or altitude for your case (65ft / 104degF) -see also table on page 4 of brochure

http://www.gaselectricpartnership.com/L%20Waukesha...

Would suspect this 5.5% derate on 5000bhp to get to 4725bhp is to account for power to run auxiliaries attached to the gas engine - confirm with Waukesha.

If that 275 hp loss is intended for auxiliaries, then the Ariel Performance file isn't configured properly. When the HP is shown like that, someone has entered a % derate for the engine - typically based on site conditions (e.g. elevation as mentioned), but could be anything else that affects engine performance without specifically being an external power draw. Perhaps fuel gas quality?

If it was entered as auxiliary power draw, it would look like this on the performance sheet:



This is what the engine configuration power section looks like in Ariel Performance software.