The '220 minus your age' formula for estimated max heart rate is easy to remember, which is exactly why it spread everywhere -- but it was derived from a small, non-rigorous review decades ago, and the actual standard error across individuals is large enough that it can be meaningfully wrong for any specific person, not just an average estimate that's slightly off.
Why heart rate reserve is usually a better basis for zones
The Karvonen method calculates training zones from heart rate reserve -- the gap between your resting heart rate and your (estimated or measured) max heart rate -- rather than from max heart rate alone. Because it factors in resting heart rate, it personalizes the zones somewhat: two people with the same estimated max but very different resting heart rates (say, a sedentary person and a well-conditioned endurance athlete) will get different target zones under Karvonen, which better reflects their actual cardiovascular fitness than a formula that only looks at age.
What still limits any formula-based estimate
Both approaches still rely on an estimated max heart rate unless you have measured yours directly (through a supervised maximal exercise test), and estimation formulas all carry real individual variation. Genetics, training history, medication, and other factors can shift someone's actual max heart rate well outside what an age-based formula predicts. A formula-based zone is a reasonable starting point for structuring training intensity, not a medical measurement.
Using zones in practice
For most recreational training purposes, calculate zones with the Karvonen method if you know your resting heart rate, treat the numbers as a training guide rather than an exact science, and pay attention to how effort actually feels (perceived exertion) alongside the numbers. The heart rate zone calculator and Karvonen calculator both let you compare zones from different formulas side by side for your own numbers.