I think this video should help answer some questions about APS bills being inconsistent, not a great answer... but something!
"With a record-setting stretch of triple-digit heat in Phoenix, Valley residents are cranking up their energy use, setting peak energy demands for APS and SRP. The uptick in demand is occurring at a time when there are also increases in costs. APS recently received approval for a rate hike of about 8%. One north Phoenix homeowner we spoke to said their July bill was $714, the highest they’ve ever paid. They also mentioned that their previous record bill of $550 was just the month before, representing nearly a 30% increase."