There is several errors in your thinking about reinvesting.

For example if you Buy say 100THs today and activate reinvest right away. You will have about 500THs after 4-5 Months.
After 366days you will start to lose your initial investments but all the later investments has probably built up a total hashrate of over 1000THs, perhaps alot more.

And day 367 you will only lose about 1.8THs but you will gain like 80THs since you already have so much.

To make this easier to understand there is a very good calculator out there that not many know about:
https://whatsmyip.com/hashflare

Have a look at that one and also when clicking the "see more" button to the right you will also get to see HashFlares own calculation.
I think that if you scroll down one year then you will fully understand what I am getting at.

so.. https://whatsmyip.com/hashflare look at it and comment if there is any questions about it.