has anyone come accross an anomaly like this before ?

I have a spreadsheet the has a 2 cells that have been merged i have not mergered the cells i received the spreadsheet with the cells already merged, both cells contain a value. One contains the value 40 and the other 65. When the merged cell is clicked the sum view in the bottom right of the window shows the value as 105.0 I have read on microsoft's site etc that when cells containing data are merged the the value is taken from the top left cell and the other data is "deleted", this has not happened with this merge. If i unmerge the cell and then remerge the cell then the cells are merged correctly and the sum does not show 105 as the values have been discarded.

I cannot replicate the incorrect merge ??