Makes sense to me, mine is the same what and I often have trouble discerning exactly where my pH sits too, but if I stare at it long enough I usually feel pretty comfortable in the end. Either way though, it wouldn’t make much difference in terms or growth so there is definitely room for error. Nothing to worry about in my opinion.
Sorry I can’t help with the actually question itself, but I believe you wont really see any beneficial growth until after 10ppm anyway and really want something more along the lines of 25ppm.