Curved vs. flat FRER. I need help.

I took a curved FRER this morning with FMU and got a super faint line. Since I've read so much about them being bad with indents I bought a flat FRER and took it on a 4+ hour hold and it shows nothing. The curved FRER says it detects as early as 6 days before your period and the flat FRER says to take the day of your missed period. So, is the curved more sensitive than the flat fast response? Does anyone know?

Top pic is my curved FRER taken with FMU and bottom pic is my flat fast response FRER.

Approx 9 dpo.