Mikey, it looks like you're talking about dKh one second, and ppm the next. You have to convert dKh to ppm...they are interchanagable like ppm and mg/L
The most common reason Kh rises in the tank is that you have some evaporation, which only removes H2O, not Kh. Even if you don't top-off to replace this between water changes, the trend will lead you to slowly raise Kh, especially if you already have moderately hard water (like me).
Thus I always recommend 50% water changes weekly for planted tanks to reset nutrients, and then occasionally I personally will do 75% changes to dilute my Kh back down to tap levels. My tap is about 9dKh, but my tank has reached 14dKh when I got lazy with water changes and just did top-offs for a few weeks.