Merging across will merge selected contents in the same row into a large cell, and keep the values in the first cell, while merge and center will merge all cells into one. Please see steps below:
Step 1: Select the cells or area you need to merge across;
Step 2: Click the "Home" tab from the Ribbon;
Step 3: Click the small triangle after the command "Merge&Center";
Step 4: Select "Merge Across" from the drop down list;
Step 5: A reminding warning "Merging cells only keep the upper-left value and discard other values" will appear, click "OK" if you are fine with the merge;
Step 6: If you have selected multiple rows, you will see the reminding message for each merge.