Question about KH and GH

[hani]

Can someone help me to undersstand, my tap water GH 4.5/KH 4, I do 50% water change on my 29g tank, i use EI, and add 5g of GH booster. my tank GH 6 and KH 2? how come the kh is lower in the tank
i use co2 30ppm
my ph is 6.6 at night and 5.7 in am
thanks