It means your soil is probably more acidic than it should be. Depends on volume of water you put in and what's in the soil.. e.g. if you guesstimate 1:1 you can extrapolate the soil's pH with a little algebra and google.
If early in grow, as in first week, add some garden lime according to instructions. That helps raise pH. There are other things you can do too. Garden lime is useless in short grows or later in a longer grow. it won't be usable by the plant soon enough.
Check the pH of your mixed fertilizer. That may be the cause if you have not done so. Most ferts are quite acidic.
Above all, if this has been consistent and you have no problems with you plants, it could be perfectly fine. If you have no problems through bloom, go ahead and test a higher pH, but if it's no different yield, why waste the effort.
I grow in coco. If i am at the recommended 5-.5-5.8 range, my plants suffer. Whether that's genetics, something environmental, or just bad "bro science" in the first place, I have no idea. Don't get me wrong. pH matters in bio-chemistry.