Calculations have no way to "know" that there is a portal filter. In fact, you can place that same calculation field on a layout without any portal at all. So aggregate functions such as sum, count, average.. ignore portal filters.
You'll need to use a different method to compute your monthly total.
ExecuteSQL can do this.
You can also change the relationship so that the current month is a value in a field in clients and used as one more match field in the relationship on which the portal is based so that you can remove the portal filter. Such a calculation field can be an unstored calculation and it will still work.
I thought that might be the reason, not sure I can change the relationship as I may want to look at last month or three months ago at some point. is there a problem of me calculating the result via an if statement i.e. if(line::Project_Month=current month;sum(lines::project_time))?
Your added match field can be a calculation field or even a field set up with a value list.
(we were "filtering" portals long before FileMaker 11 by using additional match fields.)