You are taking things too personally here. As a consultant I have been working fairly regularly in the past few years on large software projects for some of my customers. That has involved being in charge of formally setting business requirements and negotiating price and schedule for scope modifications with their providers.

I will systematically challenge any provider planning for a few reasons:

- If they fold straight away, it’s a free win for me and I have children to feed.

- I will myself pad a planning when I am in the reverse position to give me leeway in negotiation and I don’t want to leave all of that on the table.

- Keeping a moderate amount of pressure on my providers will help me in futur negotiations. I want them to leave the table feeling I made some concessions.

- Having my providers be late is extremely advantageous for me. On large projects it’s a quasi-certainty that at some point they are going to complain about scope creeps or delays in writing the specs and ask for money. If they delivered late, I have something to push back with especially if penalties are involved.

- I am also systematically negotiating hard with companies which did themselves do it. If finding a price with your company was a pain, I am likely to retaliate on scope and schedule.

If you are strictly working on the technical side, you are probably not privy to most that. A significant part of what’s happening might not even be related to you or the quality of your estimation at all.