Why does lightoom 6 not keep my custom crop size

Why does lightoom 6 not keep my custom crop size. I want to print A1 (841mmx594m) size so I set an equivalent custom aspect ratio (8.41 x 5.94) and that worked okay. But then I realized I needed a 5mm per side bleed (51mmx604mm) so I changed aspect ratio to 8.51 x 604, but Lightroom keeps changing it to 7 x 5. Now whilst very similar 7x 5 is not the same ratio as 8.51 x 6.04, i.e 851/7 = 121.57, whereas 604/5 = 120.8, so if I multiple 7 and 5 by the lower ratio I get 845.6 x 604 so Ive lost 5.5mm from the landscape length

Why does it do this ?

Edit I wonder if it just accurate to two decimal places and doesn't round up, so these are the dimensions

841/594 = 1.4158 851/604 = 1.4089 7/5 = 1.40

and as you can see the last two are the same to two decimal places if don't round up, but they are different if I do round up.

If I set the aspect to 1 x 1.41 then Lightroom doesn't change it to 7x5, and I think this is close enough as 1 x 1.41 is the same as 851.64 x 604