They recently changed how it was calculated, I myself went from 5 to 3.
Tenure is based on the total years of Xbox LIVE Gold time completed. The clock stops when you go Silver/Free and Xbox LIVE Silver time does not add (or subtract) tenure. Tenure increases only when the year of Xbox LIVE Gold time ends. For your first year
of Xbox LIVE Gold you see '0'; it ticks over to '1' year of tenure when that year completes. If you've had Xbox LIVE Gold for 3 years and 6 months, you'll see a tenure of '3' next to your Gamertag.