A bike advertised with 80km of range might return 40km for one rider and 90km for another. Neither figure is wrong, they are just measured under different conditions. Range is a function of how the battery's energy (in watt-hours) gets used up, and several factors change that rate a lot.
Assist level does most of the work
Riding on the highest assist setting the whole way can roughly halve the range you would get on the lowest setting or on eco mode. Manufacturer range claims are almost always measured on the lowest assist level, on flat ground, by a light rider. That is the ideal-case number, not the typical one.
Rider and cargo weight
The motor has to move more mass with a heavier rider, a loaded pannier, a child seat or cargo. Every extra kilogram means the motor draws more current to hold the same speed, which draws the battery down faster.
Terrain and stops
Climbing draws far more power than riding on the flat, since the motor is fighting gravity as well as rolling resistance. Frequent stop-start riding, at traffic lights and intersections, also costs range, because accelerating from a stop draws a large burst of current compared with cruising at a steady speed.
Tyre pressure and tyre choice
Under-inflated tyres increase rolling resistance, which the motor has to overcome. Wide, knobby tyres (common on fat-tyre and trail e-bikes) also cost more range than narrower, smoother commuter tyres, even at the correct pressure.
Wind and cold
A headwind has the same effect as a mild, constant climb. Cold weather also reduces range on its own: lithium-ion battery chemistry loses usable capacity as the temperature drops, so the same battery will show noticeably less range on a cold morning than on a warm afternoon.
The eBikeFinder directory lists each bike's advertised range as a minimum to maximum band (where the brand publishes one) rather than a single number, precisely because range depends this heavily on how the bike is actually ridden.