Yup, this is something that shows up¹ for melanoma, thyroid, and prostate cancers: those who are diagnosed when the cancers are in situ (haven't left the tissue of origin) have better 5-year survival chances compared to people of the same age, race, and sex. Likely because, if people are diligent enough about their healthcare to report early warning signs and get the recommended screenings, those people also have much healthier lifestyles. If there were a way to control for lifestyle, I'd imagine the "benefit" would disappear or become a small but clear negative.

1: You won't find relative net survival above 100% in the CDC's statistics. That's because they calculate survival rates using daily differences in death hazards derived from life tables of people with cancer and those without. Add up the differences across all days, do some exponential math, and voila: relative net survival rates. But, if the relative risk for a day is negative (i.e. those without cancer have a higher risk of death), then they set the relative risk to 0 instead for that day. Which is ridiculous, IMO. It's forcing a distribution of actual events to match an idealized model.