Right now, the weekly and monthly change calculations are based on the current day minus the first day of the week or month. Any weight fluctuation between the last day of the week/month and the first day of the next week/month is lost. Consider the following:
31-Jan 190 = + 5.0 for the month
28-Feb 187 = +2.0 for the month
31-Mar 185 = + 3.0 for the month
In the scenario above, if I add up the monthly changes, I'm +10.0 lbs over three months when in fact my weight didn't change at all. (The same miscalculation happens on a weekly basis).
This could either be remedied by using the last value of the previous month/week as the basis for comparison. Then the monthly changes above would be +5, -3, -2 in the scenario above, and would accurately reflect that my weight hasn't changed over the three-month span.