Target impression share of 15% despite 100% targeted share, and bid limit 10 times higher than average CPC... what gives?

Hi all. I know target impression share isn't the best strategy, but I've tried many things with CPC ads over the years, and very little sticks (including agency work) probably due to the very niche nature of what I'm selling.

I thought I'd try and do a simple 'just stick me at the top, no matter the cost' approach with target impression share and a bid limit of 5 euros (around ten times as high as the actual CPC) on around 150 exact match keywords.

Despite this, average search impression share per keyword is only 15%, with an average CPC of 0.76 euro.

Now, I understand that eligibility to display on a keyword is based on current ads' targeting settings, approval statuses, bids, and quality, etc. but that seems an extremely low share.

My previous PMAX was getting around 30% impression share, and that had a pretty conservative CPC limit.

Anyone might hazard a guess as to why this is happening? Thanks.