This is one of the more frustrating situations, because the obvious suspect has an alibi. The battery reads well, and the car still will not start. Here is what is usually going on.
Resting voltage is not a test
The first thing to question is the test itself. A multimeter across the terminals measures resting voltage, and a battery can read a perfectly respectable figure while being completely unable to deliver current under load. Capacity and voltage are different things. A proper load test asks the battery to work, which is the only question that matters.
The connection, not the battery
A battery in excellent health is useless if the current cannot get out. Corroded terminals, a clamp that is finger tight, or a corroded earth strap between the engine and the body all create resistance in the highest current circuit on the car. The dashboard lights up fine, because lights draw almost nothing. The starter, which wants hundreds of amps, gets nowhere.
Cleaning terminals and checking the earth strap resolves a genuine share of no start calls where the battery is healthy.
The starter itself
One heavy click with strong dashboard lights points at the starter rather than the battery. So does a grinding noise, or a fault that only appears once the engine is hot.
Security and immobiliser
Many cars will crank happily and never fire because the immobiliser has not recognised the key. A failing key battery, a damaged transponder, or a spare key that was never properly coded all do this. If a warning light on the dash is flashing and the engine turns but does not catch, this is worth considering.
Fuel and ignition
A strong, healthy crank that never becomes a running engine has moved out of electrical territory. Fuel pumps, filters, crank sensors, and ignition components all produce exactly that symptom, and none of them care about your battery.
Safety switches
An automatic that will not crank in Park sometimes cranks in Neutral, which points at the neutral safety switch. A manual that does nothing until the clutch is pressed fully to the floor has a clutch switch involved. Both are worth trying before you assume the worst.
What we do with this
We test under load rather than at rest, check the terminals and the earth, and measure charging output. That narrows it to electrical or not electrical quickly. If it is not, we tell you what the symptom points at and you save the money you would have spent on a battery that was never going to help.