That's not correct - as long as his base salary (guaranteed or not) combined with any amortized bonus and "likely to be earned" incentives put him in the top 51 cap hits on the roster, he's counted against the cap at the full value of the previous three items. Think about it - there are very few guaranteed base salaries in the NFL. If the non-guaranteed base salaries were not counted against the salary cap unless/until they were actually earned, there wouldn't be a team in the NFL that had a cap problem until opening weekend of the season.
For the record, "likely to be earned" is defined by whether or not the measure was reached in the previous season. Mays has a $100,000 roster bonus in his Texans contract, and I'm guessing since he made the Broncos roster last season, the $100,000 is included in the Texans current cap amount.
One other item that kind of relates to this. Mays is a vested veteran (meaning he has 4 or more accrued seasons). As such, if he is on the opening day roster, his entire 2013 salary becomes guaranteed.