This is spot on! When I first transitioned those campaigns from eCPC to tCPA there was a big learning period. As the campaigns scaled and collected more data the actual CPA started aligning closely with the target CPA, but it took some time. The more conversions you do the faster that alignment will happen. After 2+ years on tCPA (and many behind the scenes improvements to the Algo by Google), I could get that delta to match within 3-4 days of making a change to the target CPA. That was with thousands of conversions a day though, so scale absolutely helps.