Negative opk turning positive after 8 minute cut off time.

I've been getting positive opks every day since CD6, it's now CD22. I bought some different ones and used those yesterday and today and they were both negative at the 5 minute mark but positive at the 10 minute mark. The new ones have a 8 minute cut off time and the old ones had a 10 minute cut off time. The first type of tests I was using were internet cheapies and I was reading them before the 10 minute cut off time and they were all positive. Is it possible that a negative result was turning positive as an evap type line before the cut off time with the internet cheapie ones? The new brand seem to be doing it just after the 8 minute cut off time...

Hope that makes sense...