Always wondered why people run a steady level of prop when using it as a kick start in conjuction with test e or c?

When you start out at day one and do a shot of prop and test e for example the test e is not avalable as soon, but after say 2 weeks of test e shots some level is becoming available and slowly building up. Why then do you still shoot the same amount of test prop?

I thought one of the goals were to keep levels as steady as possible to reduce sides? Wouldn't dropping the prop each week be more accurate for keeping a more steady level of test available

Just a question I have always wondered?