Strange 'hole' on a wifi site survey, Extricom Blanket

Hi guys, so, as an entry-level networking guy i was checking the wifi coverage of a supermarket in my area; using Ekahau Heatmapper.

Thing is, this is an Extricom blanket coverage, so, multiple AP presenting themselves with the same mac and channel.

Problem is, under a specific antenna, i get a hole, no signal at all no the laptop that i'm using in passive mode, but i do get signal on other 2 instruments (and, a handheld scanner turned on in that location gets signal).

It's like for a 3 meter radius around the AP, my heatmapper sees no signal.

I have trouble understanding whats going on; do you guys have any pointers?

Thanks in advance