How exactly does bid incrementation work on the shard auction house?

I tried to bid on something that the current bid was 4, and it cost me 5 shards to bid, I was outbid and the new current bid was 6, given the default start bid is 3, it seeemed like it was a simple 1 shard increase per bid.

However, from that current bid of 6 shards I was going to have to place a bid of 16 shards to make next bid.

Why such a large jump after only 1 shard per bid previously?