Problem with a sites "details for today"

I have the following problem:
I used a site (a homepage; between 9pm and 11pm) which says for today (24m 57s) total. But when I click on it to reach the "App / Site Details" the graph (18m 54s) shows for 9pm and (24m 53s) for 10 pm. So the total would be (43m 47s). But I think 24m 57s must be the right time and because I used that site while the hour changed (and without a break), the 10pm graph seems to show the total time, not only how long it was used within this hour. Could that be?

I also have a screen of that where you can see the two bars (one up to 18.75, one up to 25) and on the same screen the total of 24m 57s.

supplement:
This also leads to depended tags showing the wrong overall time:
I tagged the site as "video" and "fun" and now "video time" is (1h 10m), but I used the three depending apps/sites for only (24m 57s) + (16m 57s) + (10m 12s) = (52m 6s), so there also is a difference.
And "fun" (1h 16m), but the 8 depending sites/apps only add up to (49m Xs).

So there is something wrong...