Security Screen Measurement..having a grey moment

Would someone be able to explain to me why most standard Security Screen Doors are only 813mm wide.

My door jambs are 826mm apart to fit a new standard 820mm wide door with 3mm gap each side.

Now if I try to fit an 813mm wide security door between the jambs on the outer side ...I have a large gap of 13mm or 6.5mm each side.

If I try to fit it between the door stops which are usually 12mm or 11mm thick then I only have an 804 mm width gap which is too narrow for the screen.

Am I missing something here as I don't think I should have to pack out the door frame to fit the 813mm wide screen door.

Why isn't a standard screen door 820mm wide to match the standard timber door width ?

Because it usually goes on the outside, which is the narrow part of the jamb set, usually is about 795 or so opening, more if there’s architrave on the outside. But, there usually is a quite big gap on most of them, not sure why.
the only time you would want an 820 one is if you had a double rebated jamb set, with a 35 or 42 mm rebate on the inside, and a 20 mm rebate on the outside for the screeen, but it needs to be a wide jamb so that the door furniture (locks and handles) don’t clash together.

I am replacing an old metal door frame with timber so my door jambs are a constant 38mm thick and are not rebated to be narrower on the outside.

Should have planned that a little better as I did have to reduce the original thickness of the timber from 42mm so instead of reducing the thickness overall with the thicknesser to achieve the required 826 gap width for the door, I should have just rebated the door jamb on the door side and kept the 42mm thickness on the outside for the security screen.....too late now, I will find a work around.

As it is, I may have the get the security screen measured and made to order as the original metal frame did not have a door sill and I have added a sill which has reduced the door height to a non standard 2022 instead of 2040.

Lessons learnt.