[Help] strange results in life span test

When I begin to collect all my life span data, I found a big problem. In all the tests, the mean life span of normal N2 control group are 25.64days, 24.72days,20.19days ,17.95days and 26.64days separately. Only the 17.95days was accordance with the public data, the others reached the longevity line.
I am looking for ways to solve the problem.
Could you give me some pieces of advice?
How to explain and which facts should I pay more attention to solve such a problem?

Thank you!

How old is the N2 sample you are using?
If you have propagated your N2 for a long time in the lab, it could have accumulated changes.
Perhaps you have to go back to a good old frozen stock.