One aspect that might explain the differences is the TimeType property in the filter.
If your Get-TaskPlus is referring to my Get-TaskPlus function then in there I used the [vmware.vim.taskfilterspectimeoption]::startedTime
The other options are queued and completed.
It is possible that the Web CLient uses another type then startedTime.

Do all the StartTime values in the returned Tasks fall within the specified interval?