You also didn't account for the fact that MANY candidates in the express entry profile are making it to the ONE-year work experience in Canada that adds 50+ points depending on your level of English. Personally, I've just earned skilled Canadian work experience (12-months) few days ago and updated my EE profile. CRS score jumped from 410 to 494 points! I'm so excited and ready to receive the long-awaited invitation to apply email.
Another reason it may not go below 400 or even 410 is because of the fact that many provinces like Ontario, a major player in the Canadian Immigration System, have programs such as the "Human Capital Priorities Stream", which selects candidates who achieved 400+ points in their EE Profile for Provincial Nomination. This program is a revenue maker for Ontario as each applicant pays around $1,500 CAD to be nominated by the province. Therefore, rounds of invitations below 400 or 410 is a conflict of interest between the Federal Government and individual provinces as it undermines their role in the immigration system and cuts back the funding for their Provincial Nominee Program.
Hence, as much as Canada loves new skilled immigrants, it is worth considering the business-side of the Canadian Immigration System as they're a source of funding and incentives for Provinces to create/maintain and innovate their individual immigration programs.
Regards!