Recently, I was trying to understand the details of how grid transforms function in Slicer and how transforms generated using ANTs in Slicer differ from those generated using ANTs at the command line. In addition to the issues that @lassoan very nicely explained above, there are a few other subtleties that I found confusing and took me a while to sort out. I am attaching my summary notes from that exploration in case they will be useful to you or others coming across this thread in the future.