It is indeed related to the increasing total network hashrate.
https://bitcoinwisdom.com/bitcoin/difficultyBitcoin Difficulty: 5,006,860,589
Estimated Next Difficulty: 6,135,992,123 (+22.55%)
Adjust time: After 158 Blocks, About 20.9 hours
Block Generation Time(?):
1 block: 7.9 minutes
3 blocks: 23.8 minutes
6 blocks: 47.6 minutes
With blocks being found at a faster rate (on average), transaction will get confirmed at a faster rate (on average).