One thing I am slowly learning...ALWAYS check pH. 6.5 is too high unless you are using really acidic soil. 6.3 is often said to be golden, but I have found 6.0 to be best. As salts build up I would rather go with 5.9 than adjust and risk 6.4.
Are they all the same strain? What is it? Are there any other variables? One pheno of Red Diesel does this no matter how dry you keep her and I grew OG Kush that did it also (1st grow...who knows...but pH would be a good guess)
Like others said, some are just bitchier than others.
Oh and as for tap (I would absolutely not use unless It was very low in ppm) mine usually is 7.6 and ~280ppm. I was rinsing my meters cuz I am out of flush water which I usually use, and my water is now pH 6.5 and 670ppm! I even checked and calibrated both meters because of this. I read that the composition of municipal water varies due to influxes of storm water which needs more treatment and seasonal changes of water source(s).
Unless you have really good water, pay $0.25/gal for RO. Make the dissolved solids in your nutes the ones you want, not water disinfectants and toxic amounts of nute/mineral locking calcium...just my lousy opinion though...wish I had good water...rainwater.