The difference between those two values could not be significant. If Sbet of support and catalyst values were within R2 of analysis - its ok (e.g. 500 m2/g and 510 m2/g, while R2 = 0.95). At low metal content (up to 1-2 wt. %) it can be the case. If not - a) (most likely) there is a technical problem with the device or experimental procedure b) you need to find out how the active phase creates additional surface (extremely unlikely scenario)