Ovulation sticks give different readings

Just wondering if anyone else has has this problem. I've used the clear blue digital ovulation stick two months in a row and got the 'smiley face' to how high fertility. But at the same time, just using traditional drug store tests, they haven't come up with a double line to show higher LH levels. Could anyone help explain this? I'm TTC for the first time and feeling a bit overwhelmed with all the tracking equipment.