Ovulation dates off

On another application it had me ovulating on the 11th (an application I've used since 2010 minus when I was pregnant) I started testing for ovulation using answer brand 20 count on Sept 30th with negative results until October 6, 7 and 8th then back to negative.  I'm confused as that's way off from when the application predicted. Is that normal? This is my first time using ovulation testing. I am not bbt or checking my cervix. I had cramping on October 6th and then some last night. Light cramping that went away after 10 minutes or so.