It has to do with the proximity of the numerals to the the ground. Yours is a tall date. Tall date - Click Here Medium date - Click Here
Because the coin has been damaged for lack of a better word. Look at the reverse, the usual cause for that is tape. And if there is even the slightest trace of it on the coin the TPG's would not slab it. They may call it environmental damage, altered surfaces or one of several other things. Some might not even slab it because of the 2 gouges on the obverse.
I just received my coins from ANACS... The 1846-O came back a medium date instead of a tall date (dang) graded EF-40 DETAILS. The slab also has polished on it, maybe someone could clarify what this means? I also had an 1877 come back EF-45 cleaned.
If yours is a medium Date then this so-called Large Date isn't a Large Date: http://cgi.ebay.com/1846-O-Tall-Dat...ryZ11970QQssPageNameZWDVWQQrdZ1QQcmdZViewItem Ribbit, Toad :smile
The difference in the dates lies in the upper loop of the 6. The coin is a medium date. The one that is on ebay is the tall date. See the upper loop in the 6.