Uneven Tire Wear

I had my front tires replaced today. I used the same tire (Michelin Sport AS 3+). The old ones had around 25K on them and were at 4/32. I probably could have held out a bit longer, but the inside edge of the left tire was worn bald. I assumed I had an alignment issue, but they checked and said it's good. The mechanic suggested it could be due to low pressure but I check it regularly both via the TPMS reading and my own gauge. I am a bit perplexed about why this happened. I've not had a tire wear so unevenly like this without an alignment problem. Any thoughts?