If you have at one point changed the blocks in the front-end, your preferences will have been saved.
For instance, let's say you have selected block IDs 1, 2 and 3 as default. In the front-end, you change the left column to block ID 4, but leave the other two as-is. Even if you change the default of the middle block, this will not be reflected on your account.
To properly test this, open a new browser as a guest