Shouldn't these two types of queries return the same results? They don't always. Why not?

Attached is a zip code database you can use to test.

Here are two queries:

> use geo
> db.zips.find({'loc': {$near:

{x: 40.752315, y: -73.977842}

, $maxDistance: 0.01}}).count()
3

> db.zips.find({loc: {$within: {$center: [ [40.752315, -73.977842], 0.01 ] }}}).count()
25

But...

> db.zips.find({'loc': {$near:

{x: 40.752315, y: -73.977842}

, $maxDistance: 0.011 }}).count()
26

> db.zips.find({loc: {$within: {$center: [ [40.752315, -73.977842], 0.011 ] }}}).count()
26

If they're supposed to return the same results, why have them both? If they're not, what's sort of meaningful distinction is there between them?