3 Unit Listing vs. 2 Unit per Cook County Assessor

[Felix Z.]

Hello All, Long time lurker, 1st time poster and soon to be first time real estate investor. I will be viewing a property that is listed as 3 units. Per the Cook County Assessor, this building is only 2 units. Should I be under the assumption that 1 of the 3 units is not a legal unit? ...Or could this be an oversight on the assessor's part? 

This building is being listed as "Recently renovated", a City of Chicago building permit reveals a permit for interior renovation in late 2012. 

Thanks!