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x:=(@Today - @Created)/86400;
x < 1; "New";
x = 1; "Open 1 day";
x <=5; "Open 1 to 5 days";
x <=10; "Open 6 to 10 days";
x <=30; "Open 10 to 30 days";
"Open > 30 days")
From a load on the server point of view, you'd be better of setting a field in the document to track the number of days open, as using @today in view column formulas leads to poor performance, and unnecessary load on the server, basically a poor design practise.
Other things that may lead to what you're seeing. Are any documents response documents? Do you have a response column in the review? Is response hierarchy enabled in the view?
Feedback response number CTYRAYMS25 created by Carl Tyler on 05/10/2018