laser cutter - inconsistent results.. ideas?

Of the four cuts i made (all were the same diagonal 6cm line to use two axises at once, about , only one of them made it clear through (my first attempt, beginners luck perhaps)? even though the first and last cut used the same settings.. 

What's even more puzzling is the kerf on the first cut is really thin (which i like) where as the other 3 test cuts' kerfs are 3 times thicker.. 

-- process :

after drawing a test line in lasercut, i placed the board in the upper left hand corner of the bed (since that's where the head is parked, at the origin)

- first cut (based on the wiki for my material) - speed 10, power 100%, corner 80% - result: clear through, thin ~0.25mm kerf

at this point i thought, wonderful, wiki was spot on.  I wonder if i can slightly speed it up to overall reduce cut times..

- second cut, speed 13, power 100%, corner 80% - result: not clean through (only about ~8.5mm through.. ), but the kerf now is roughly three times thicker, ~0.75mm.. strange

ok, guess i was too over ambitious let me dial it back a bit.. 

(flipped board over but instead of top left corner of bed, i placed it closer to the centre)

- third cut, speed 12, power 100%, corner 80% - result: not clean through, barely 8mm deep a cut.. kerf again is on the thick side

hmm, well maybe i should just stick to the exact settings in the wiki (usually a good idea i'm sure)

- fourth cut, speed 10 again, power 100%, corner 80% - result: thick kerf again AND not all the way through (only about 8.5mm)..

see attached photos for info about the resulting cuts outcomes

--- 

at this point i'm baffled.. my questions:

- why are the results so inconsistent, using speed of 10 worked first time but not second ? Patrick, who was sitting near by, advised me that we should NOT go below 10.. does the cutter just not cut then ? i remember a thread a few weeks ago about going below ten but don't remember if that was speed or power %.. 

- does it have to do with where on the bed i'm placing it? Patrick theorized from his MDF experience that not everywhere on the board has the same amount of lasering energy reaching.. so where on the bed should i be testing so an entire A0 size sheet gets properly cut?  

- why was my first kerf all lovely and thin? guess it's because it went clean through so less material was burned away.. i would love to get that kerf thickness again if possible.. 

I just hope my first test wasn't a fluke :-\

I have a few sheets of 9mm birch ply i would like to cut, so i brought into the space today around 2pm a small test board, 20x32cm test board to test/validate settings to use..