What happens if you put the micro sd back into the laptop after some video is recorded to it? Is the full capacity still there or does it show the 100mb missing? If it's missing, do you see any files that may be taking up that space?
I know in the past I tried transferring files from a different brand DVR to a USB Flash drive and the size wasn't adding up right. Once plugging into the computer, I noticed that the DVR was automatically putting an installation file for a video player software on the USB drive that I could use for playing back the video files on a computer...It's possible the camera could be putting some type of software or other files onto your SD cards, so that would be worth checking.
What Jack is referring to is there are a lot of counterfeit SD cards on the market (some even with popular brand name logos printed on the cards) that are actually smaller capacity cards that are hacked to display more space than they actually have. For example, it may only be an 8GB card, but is sold as, say, a 64GB card and hacked to display 64GB on a computer when formatted. He's probably assuming this happened to you and your computer is displaying more space than the card actually has....But if we're only talking 100mb, this is likely not the case, as it wouldn't be worth the trouble of hacking a card just to make it display 100mb more. I think your case involves either formatting or extra files being dumped onto the card from the camera's internal memory.