There is frequently a significant discrepancy between the time a player is predicted to be on the bench or DL following injury as compared with actual time lost. This is because the nuances of each injury and the variability of both healing time and progression through rehab are often difficult to anticipate.
A need to address the influence of predisposing factors (such as muscle imbalances or deficiencies in strength and flexibility, or even mechanical flaws) may also alter the equation, as would comorbidities (the presence of secondary diagnoses in addition to the primary condition). Sometimes the extent of the issues posed by these secondary problems is not completely clear in the initial phases of recovery... Then there are also the complications during recovery that may intervene.
There are other major factors at play as well -- for instance, an injury that is recurrent, generally indicative of greater vulnerability or chronicity (unless due to repeated trauma), might necessitate a more cautious approach. Or, as with injuries that don't recover sufficiently with conservative management, surgery might be required even after an initial attempt at rehabilitation.
In pro sports there is often an incentive to play, even at the expense of pushing beyond one's limits or following injury. One of the more confounding things that contribute to the discrepancy between playing time actually lost and that for predicted time lost, is an accelerated return to competition. Competing before readiness truly allows may bring on new problems in addition to possibly exacerbating the original issue. Yet another factor, and one we've seen curbed somewhat across sports, at least with respect to concussions, is playing hurt. We see this in football all the time, particularly as teams make playoff runs or compete in the post-season. Generally there is a price to pay. Sometimes it is simply a short-term price paid in performance, but playing hurt often results in prolonging recovery, heightening injury or translating to long-term issues that can affect quality of life. This clearly applies to orthopedic as well as neurological conditions.
So, when you read injury reports and see a player will be out of action for an estimated length of time -- take that with a big grain of salt. Don't let your fantasy team depend on it.