Edit: you are right, this is a techsupport related question and should be asked to them, but I'll answer it this one time.
Would you be so kind and try that while the trend is selected?
I remember that "one period" needs a value to be pick from an element, if it has no elements it defaults, so does the scroll bar, perhaps the scroll bar default is wath you expect but the default for the one period back is not!
However, one period back does create a period based on the selected element, if you try that with the trend being selected you should observe the expected behaviour.
Please try this out and post your results back, so we can get a general idea as what's going on.
And do send this to techsupport.
Posted it to Tech Support at same time as my original post, who confirmed this is a known issue and will be fixed in a future release of ProcessBook. At least I wasn't going crazy and it is a bug!