You are quite right it doesn't. He doesn't say that immediately after the turn from Main onto Houston, he heard 2 shots. They are two separate sentences. Elsewhere he says that he heard the two shots as the limo passed the TSBD. From his W.C. testimony.
"Mr. WILLIAMS. After the Presidents car had passed my window, the last thing I remember seeing him do was, you know--it seemed to me he had a habit of pushing his hair back. The last thing I saw him do was he pushed his hand up like this. I assumed he was brushing his hair back. And then the thing that happened then was a loud shot--first I thought they were saluting the President, somebody even maybe a motorcycle backfire. The first shot--there was two shots rather close together. The second and the third shot was closer together than the first shot and the second shot, as I remember."
From his statement made 11.23.63
“There he joined two other men known to him as Hank and Junior. They were looking out of the window on the South side of the building approximately at the middle of the building and saw the car of President John Kennedy come North on Houston Street and then make a turn going West on Elm Street down into the triple underpass and passing directly in front of the Texas School Book Depository. While they were watching this car pass, WILLIAMS heard two shots which sounded like they came from right over his head."
Hardly time to adjust his testimony as it was made the day after the assassination.