Don't think it is anything to do with copying from an SD card, it's just that the Windows copy process does not count very effectively.
For example, I get very similar effects if I copy a file from one part of my hard disk to another part of the disk. I just tried copying a 6GB file. Started off (as quickly as I could get it to show the detail) at 277MB/sec, then dropped off over time to 63MB/sec.
I guess it is down to things like rounding errors in the way it counts and maybe counting the read cycle (rather then the write cycle or the complete cycle) and so in the initial seconds a lot of buffering goes on which makes the transfer seem faster than it really is.
Bottom line - don't worry about it.