You are correct that "ACB at the time of purchase" is more intuitive. The CRA adds "at the time of disposition" to remind taxpayers to include all costs and improvements made throughout ownership, not just the purchase date.
Official CRA Link: CRA - Definitions for Capital Gains
Proceeds of Disposition: This is simply your gross selling price. The CRA uses "disposition" because it covers sales, gifts, or even destruction of an asset.
Adjusted Cost Base (ACB): This is your Total Purchase Cost. It is called "adjusted" because the original purchase price is "adjusted" (increased) by any costs to acquire it (legal fees, commissions) or capital improvements made over time.
Hope this answers your question, if you need further clarification, you can always contact CRA directly.
