Why is this RAM not compatible with my Dell Latitude 3640?

Hi, I tried to put 4GB RAM from my Acer Travelmate P236 into my Dell Latitude 3640, but it wouldn't work. Is there any reason for this? 

I understand RAM is not always compatible, but they seem very similar.

I put it in, switched it on, but it made very loud beep warning noises, so I turned it off and removed it.

Underneath the top RAM it says ONLY DDR3L DIMM A. The bottom one says ONLY DDR3L DIMM B, as you can see from the first picture.

In the bottom picture, the RAM at the bottom is the Dell RAM, and the top is the Acer RAM. What is the difference, and why did it not work? Thanks so much for your help.