I don't claim to understand anything, but the point of the
GUID partition scheme, which is now the default (or supposed to be, but YMMV) on Intel Macs is that it is possible to repartition without erasing the disk. You are right that traditionally repartitioning effectively deletes. I've also noticed that selecting the repartition tab in Disk Utility, even (I think!) for GUID partitioned disks, gives me the warning that repartitioning will delete the disk. So I don't know the procedure for using Disk Utility to repartition losslessly; I only know that I've read people here who claim to have done it successfully. This is also why I encourage you to back up.
I have experienced, however, that Boot Camp Assistant repartitions losslessly. And via the
Winclone website, I learned that the
diskutil Terminal command now has a
resizeVolume option which purports to be lossless and so is surely what Boot Camp Assistant and--if you can convince it to use
resizeVolume and not, say,
partitionDisk--Disk Utility uses behind the scenes to nondestructively repartition.