Dimensions not adding up

Hello all I'm new to all this so go easy! I have created an outline of a large shed and added dimensions (so far so good) but the dimensions top and bottom don't add up. For some reason the right hand "cut out" reads 2390 and should read 2400. (it was copied and rotated from the left hand one when creating the drawing so the drawing should be symmetrical). Surely if I had made some sort of drawing error the top figure should read 19,390! The dims shown are an impossibility! What is going on here... and can anyone help me correct this? Cheers

You can start by simply selecting the lines and watch the information at the bottom status bar.
There you find the number and length of selected entities. Select one by one to verify their correct length.
When all lines are correct, start zooming in and check the dimension end points.
Do they match with the drawing?
Possibly you didn't use snapping and misplaced the dimension start or end point.

There is also Tools => Info => Distance Point to Point, which you can use to measure any distance in the drawing.
Look at the command history box for measuring results.
Use Snap tools to ensure that you really catch line endpoints. Possibly you have to deactivate Snap to Grid to avoid irritations.

Another option is to select the lines in question and use Plugins => List entities.
This shows information about line length and coordinates.

Thank you so much for your prompt response.
I definitely had "snap to endpoint" switched on but... It is possible that I accidentally had something like "snap to grid" on at the same time.
There is lots of useful information in your reply that will take me a little while to figure out - thanks again for the comprehensive answer.
I will go have a play with your suggestions and respond more fully then