Question: Retained revenue understanding.

[jstnjy]

Hello,

I just started reading The theory and practice of enterprise AI. However, I got confused on the following formula and the explanation in the book:

It says there that the retained revenue is $280 ($100 + $180). However, the example given are the following:

1. Customer 1 spent: $100 and $150
2. Customer 2 spent: $200 and $180

So, how come that the retained revenue has been computed using the time t-1 of the customer 1 ($100) and the time t of the customer 2 ($180)?

Can anyone help me with this?

Thanks