How do brokers calculate *estimated yield*?

In portfolio summary of monthly statements of TD Ameritrade,

1. There are estimated yield percentage and yearly estimated income for stocks. How exactly do they calculate these values? Do they just look at the next month's announced yield % for one of them, and multiply that by 12 for the other one?

2. What does the total estimated yield percentage refer to exactly? Cause sometimes stock dividend appears as 3% but total is 0.0% at the bottom of the column

Note that for some stocks dividend yield changes every month.