You include above grade and below grade sq. feet without deferentiation. How come? Is it misleading?

The data that Zillow uses comes from varying sources: public records, MLS data, 3rd party syndacation sources etc. All have different guidelines for what is considered living space and only some differentiate between above & below grade depending on the city/town, mls data age & syndacated data standards. Zillow doesn't originate any of the data it comes from feed data that they compile and unless it's specifically enumerated in the data stream there's no way for Zillow to know what's being considered.

The only way to improve this data is to claim your property and correct the data to reflect your properties stats,  talk to your local municipality to improve the quality of their assessors database feeds and make sure your real estate professional is properly disclosing sources of living area (in the last 2 years the local MLS here in Greater Boston MLS-PIN now has a field to disclose if below grade finished space is included). Different MLS's provide slight differences in their data feeds so that can also be an issue.

Hope that helps,