How is obama 44th president but his wife is 46th first lady?

Two presidents had two wives while in office due to death and remarriage:

John Tyler (10th president): wife Lelita Christian Tyler died in 1842 (he had 8 children with her) and he then married Julia Tyler in 1844 (and had 7 children with her). He was in office April 1841 to March 1845.

Woodrow Wilson (28th president): wife Ellen Louise Wilson died in 1914 (he had 3 daughters with her) and he married Edith Galt Wilson in 1915 (no children). He was in office from March 1913 to March 1921.