It's possible you have evaporation going...if the solution was perfect and the plant drank and ate the same ratio if the water was just evaporating away the solution would get more potent over time...so say ya got 100parts of x in a 10 gallon solution if ya lose 5 gallons to evaporation you'd see you'd see 100 parts go up to 200 but I've heard a lot of people say that they notice the ppm slowly rise once they hit flower...if that's not the reason it could have something to do with the ratio of nutes the plants eat compared to what yer feedin this could leave a bulidup of a certain nute...I guess ideally the solution would stay the same strength possibly get just a little bit stronger (ie...a bit higher ppm from the point of a res change to the next)

As long as your plants look/feel healthy I'd say you are fine though besides a few of the leaves looking a bit curled down which is somewhat to be expected anyway once you get deeper into flower...from what I can see in the picture they are doing well for day 20 depending on how ya deal with those fan leaves you might be able to get 3z's per plant

Oh btw an easy way to test the evaporation idea is to just leave a bucket with the same amount of ventilation or as similar as possible to your res and see how fast it's drying up...my room is pretty cool it only gets to 75 or 77 during peak day I have 9 plants avg 10 inches tall still mix of male/female till I can tell the sex as I've just started bloom...but I go through a gallon per day