Battery-life figures are often legitimate measurements taken under a specific test routine. The problem is assuming that routine matches your own use. A useful comparison starts by finding the test conditions.
Look for the workload behind the number
Video playback, web browsing, standby and mixed office use consume power differently. A manufacturer may publish several figures for the same device. Compare like with like whenever possible.
Display settings matter
Screen brightness, refresh rate and always-on display features can be major power consumers. Tests run at lower brightness may not represent outdoor or brightly lit use.
Wireless radios and signal quality affect runtime
Cellular data, GPS, Bluetooth and Wi-Fi use power. Weak network signal can increase radio activity. Travel days can therefore produce very different battery life from home Wi-Fi use.
Battery capacity declines over time
Rechargeable batteries age with cycles, heat and calendar time. A new-device test is not a guarantee of identical runtime years later. Check whether the battery is serviceable and what replacement options exist.
Fast charging changes the usability equation
A device with moderate endurance but very fast, convenient charging can be easier to live with than a longer-lasting device that needs a proprietary slow charger. Consider the whole charging routine.
Bottom line
Battery claims are most useful as standardized clues. Find the test conditions, compare them with your use and plan for battery aging.