Recovery is the real gap
A truck-mounted unit generates vacuum from an engine in the van and sustains it across an entire house. A rental machine runs on a household outlet and produces a fraction of that lift. Both put a similar volume of water into the carpet. One takes most of it back out; the other leaves the majority in the pile and the pad. That surplus is what causes slow drying, residue and, in bad cases, odour from a pad that never fully dried.