Colleges report both 4- and 6-year rates. 6-year rate is commonly seen as a proxy for students who graduate period (since only a few students graduate but take more than 6 years).
However, most graduation rate differences are due to student characteristics. Entering academic credentials are likely the dominant factor, although financial situation in relation to affordability of the college is a significant factor as well (cost is a big reason for dropping out, and needing to take lighter course loads to work more hours slows progress to graduation). So it is no surprise that highly selective colleges that have mostly students from wealth and which are rich enough to give good financial aid to the rest have the highest graduation rates.