I think you guys are rather over complicating the situation, the reason is actually quite simple and logical.
Most runway specific take-off data used by airlines uses an "unbalanced" field, ie. TODA is greater than TORA because the clearway can be included in the distance required to reach screen height giving a higher RTOW. JAR-OPS (now EU-OPS for most) forbids the use of clearway when the runway is contaminated so if you enter the tables with a RTOW that used it you must make a larger decrement than if you had used balanced field take off data.
You need to check the runway page as you do the initial calculation, if TORA is greater than TODA you are using unbalanced performance and you must use the larger decrement. If TODA = TORA then the performance is balanced and you can use the smaller decrement.
If, like the original poster, you are non specific performance you may well find that the best weight is achieved by not accounting for the clearway at the initial stage and using the smaller decrement which is why the specific pages that some airlines produce for contaminated runways normally produce a better weight, they have discounted clearway at the beginning and use a smaller decrement.