Ok, I'm putting this here for posterity as I looked for answers, could't find any and finally found the answer by myself after much sweat!!!!

Let's say you are in the resource usage view and you are zoomed to quarterly. Then, for some quarters, you see red numbers but, curiously, on that quarter you do have some availability !?!?!?!

The answer is that RED does not mean that you ran out of availibility for the period you are zoomed to, BUT it means that there is a day somewhere in your period where you are overallocated. So you have to drilldown by zooming in to spot the culprit day !

CQFD