That's not a "true scope" because it only looks at half the picture. A "true scope" would include both quarterbacks' passer ratings -- yours and your opponent's.
This is a true scope.
Opposing QB's Rating
(aka Defensive Passer Rating)
2010-13
1 Seattle 75.3
2 Pittsburgh 76.6
3 Baltimore 77.1
4 Chicago 77.8
5 NY Jets 78.0
6 San Francisco 79.3
7 Arizona 79.6
8 Green Bay 80.2
9 Cincinnati 81.1
10 Miami 82.3
11 Kansas City 83.2
12 NY Giants 83.3
13 New England 83.8
14 Cleveland 84.2
15 Buffalo 85.2
16 Houston 85.7
17 Atlanta 86.0
18 St. Louis 86.4
19 Philadelphia 86.8
20 Detroit 86.8
21 Tennessee 87.0
22 Denver 87.2
23 New Orleans 87.2
24 Carolina 88.0
25 San Diego 88.4
26 Tampa Bay 89.3
27 Washington 89.8
28 Jacksonville 92.1
29 Oakland 92.2
30 Indianapolis 92.3
31 Dallas 93.0
32 Minnesota 96.2
From 2010-13, Dallas had a 25-26 record (the "losing record" you referred to) in games that Romo started and played at least a half.
According to passer rating, Romo ranked 6th in the NFL from 2010-13. This means that, with an average-to-good pass defense, the Cowboys would have averaged no fewer than 10 wins per season during that span. Instead, Dallas' pass defense ranked 31st -- far from average. Combine Romo's and the defense's rankings of 6th and 31st and you get 18.5, which puts the expected record at about the 18th- or 19th-best.
That's where the 25-26 comes from.
It most definitely does not come from lost QB fumbles that don't count in passer rating. Look how Romo's lost fumbles compare to the five QB ranked ahead of him over that time period.
Lost QB fumbles, 2010-13
Rivers 18
Manning 9
Bees 7
Romo 7
Brady 6
Rodgers 5
From 2010-13, Dallas had the #31 defense according to passer rating. This means that, with an average-to-bad QB, the Cowboys should have averaged no more than 5 wins per season, like Minnesota, Washington, Oakland, Jacksonville, and Tampa Bay did over that span.