You must be using a different metric for "top 50".
For example, you might be using top-50 in receptions. Which is not a good idea, because, for example, an injured top 10 receiver would not be in this list.
My suggestion? Find a decent list of the top 50 WR's in the NFL. For example, if you want a large survey of the top 50 WR's, here is one. Over 250,000 NFL fans voted in this survey, so the results are likely relevant as it has a large sample set.
In this survey of a quarter million NFL fans, a little more than a handful of the top 50 WR's were taken round 4 or later.