It may be a mapping error in the original subdivision. Or it may be a mistake in your neighbor's recent survey.
It's not often that someone would want to get to the bottom of an apparent error that works out in one's favor. Kudos to you for wanting to ensure correctness and avoid potential problems down the line.
Find a local surveyor who specializes in boundary work. Explain the issue to him/her as you see it.
Being a corner lot, there are several things that might contribute to this discrepancy.
1. You may not be properly accounting for street width in your measurements.
2. The original surveyor started laying out the lots of this block from the other end, leaving any excess or deficiency in the lots at your end, and by the fortune of random errors and mistakes in the original survey, you ended up with an additional 10'. This isn't all that uncommon, although such errors normally are in the magnitude of a couple feet or less in small lots. But I've seen such mapping errors in excess of 10', so it's possible.
3. Your neighbor's surveyor was less than fully diligent in his/her retracement of that lot.
4. The Assessor's records are wrong in that whoever took the dimensions from the original subdivision map read the dimension wrong. A map that old was drawn by hand and may have a poorly written or otherwise difficult to read dimension on your lot.
There are more possibilities than I can think of off the top of my head. A good surveyor will be able to properly sort it out.

