When brushing, you can reach blocks that go further than you building/mining reach. This behavior is sometimes inconsistent as you can only reproduce this by brushing any reachable block then facing slightly away to another block further than your block reach.

How to reproduce

1. Place any block in front of you, with 4 empty blocks in between.
2. Start brushing the upper part of the block (should be reachable).
3. Now slowly face towards the lower part of the block, while still brushing.
4. Notice how you're still able to brush, even though it's out of reach.