Metrics for query in v$sqlarea get reset while still in cache

We are trying to measure performance of several queries in our system. We're capturing data from sqlarea every 30 minutes over the course of a day to measure buffer gets, disk reads, etc over time. However we have a handful of queries who's values suddenly get smaller part way through the test, as if they were resetting. First_load_Time, version count, plan_hash, etc all remain unchanged. The queries execute pretty consistently the entire test. It does not seem like they are falling out of cache, but I cannot explain why the metrics all get reset. This make's it difficult to measure