To me it sounds like you did do some rescoping of work: if the number of engineers was fluctuating, that's the buffer you used because you might have misestimat

To me it sounds like you did do some rescoping of work: if the number of engineers was fluctuating, that's the buffer you used because you might have misestimated. Also, did your "first delivery" include exactly everything that was part of that initial estimate? I would imagine your clients figured something else is higher priority and that pushed out some of the initial plan: if so, that would also be rescoping work. Again, I don't think meeting deadlines is impossible, but that you get there by reprioritizing and rescoping work as you go along. And sure, resizing the team too. On top of that, even if you really did deliver everything you signed up for 6 months ago (and not more — we don't want to be overestimating/underpromising) without fluctuating capacity or scope, most in-house teams do not get 1/6th of the time to estimate something before they start on it (effort sizing is mostly useful to decide on what to do, and then that time is best spent building it). And perhaps they couldn't do it even if they did get the time.