Two mistakes, one after the other.
When I cut the shelves for the base cabinets of the hutches, I omitted the sub-top shelf on which the top shelf will be screwed on.
Since I didn’t have enough material, I had to make a supplementary glue-up with scraps, biscuits and glue.
My second mistake was my misinterpretation of the tape measure reading. I cut the board at 15-3/8’’ in width instead of 15-5/8’’ ( I should have used a story stick). To overcome this issue, I added/glued a 1-1/2’’ middle stip between the two sub-shelves.
Click here to see all the details on how I worked around these mistakes,