why is this microcontroller brown-out level detection at 2.6V and 3.4V
Common logic voltage levels are 3.3&nbsp;V and 5&nbsp;V, and many batteries have 4.2&nbsp;V or 4.1&nbsp;V. So the brown-out levels are obviously designed to be 0.7&nbsp;V below these supply voltages.

In general, power supplies are assumed to have tolerances of ±&nbsp;10&nbsp;% (this is why the datasheet often says "4.5&nbsp;V&nbsp;–&nbsp;5.5&nbsp;V"). And when your power supply outputs 4.5&nbsp;V, you want to have some additional safety factor so that your microcontroller does not reset at the slightest dip, but only when the voltage is so low that the power supply is (apparently) completely toast. (The decoupling capacitors buffer the voltage, so a dip of 0.7&nbsp;V indicates that the actual power supply is already lower, and that the decoupling capacitors are already partially discharged.)

In other words: 2.6&nbsp;V is the correct brown-out level to use for a nominal 3.3&nbsp;V supply.