DWC: Sudden Increase In Water Uptake (day 38 of flowering)

Hey RIU... I'm in day 38 of flowering my HSO Blue Dream (grow journal here: https://www.rollitup.org/grow-journals/705725-first-grow-ever-hso-blue.html) and while I normally check my progress daily, last night I was unable to check on them. I have two plants each in their own 14 gallon DWC (they hold about 6.5 gallons before getting the bottom of the netpot wet). Typically each plant takes somewhere from .75 to 1 gallon per day. They way I have things set up, I don't have to add back every day... I can skip a day. So I went into my grow tonight to see how things were, expecting to have to add back some to the reservoirs... I usually mix 3 gallons of soup and add back from there and/or plain tap depending on the ph and ppm readings... But today when I went in, each reservoir was down almost 3.5 gallons. No harm done (I don't think)... In my buckets, the roots will still be wet even if the water level is lower than desired.. But the sudden increase in water uptake confuses me. This is my first grow... So it's obviously my first experience with hydro too...

Is this normal?
Is this a sign that my sisters are stepping up production for their final few weeks of flowering?
PPM was high in both buckets, while ph was low (5.3 in one, 5.4 in the other), is there a deficiency?

Obviously, I'll be checking them daily without exception here out, so this shouldn't happen again, but should I expect the water uptake to stay the same (or even higher) going forward, or are plants like people: they just get extra hungry/thirsty from time to time?

Thanks for all your help, experts!