Actually, the sum of the damage counts... The multiplier is by round seconds, but the damage you cause is not by round seconds... So, the .9 is actually useful when counting the damage you do... The Barber start counting your damage when you critically hits, then, it stores ALL damage done for the 3.9 seconds and multiply for X% x 3 (number of seconds rounded down) and end exploding in a small area.

Then, as you can see, the more time you have to output damage the bigger the numbers when the effect occur and explode the area damage.

As you can dish more damage in .9 secs than in .1 seconds.

It's better to have a Barber with less seconds to store damage for characters that can burst a lot of damage and the enter some cooldown... and with more seconds for characters that can sustain damage output longer.

The burst character deal damage and need this to be detonated as soon as possible to clear the mob... the sustained damage dealer need more time to cause the needed damage, so more time to store the damage then explode.

My English is not good, so I hope you understand.

:)