Feature importance gbm and caret

I am using the caret package for GBM predictions and comparing them with the GBM function, from GBM package. When I plot the feature importance from each model (caret - varImp - and GBM - summary.gbm), the results were very different. Besides the difference in importance value, the features between both models were completely different, too. Does any one know why? Is correct to use the function varImp to observe this, or I can just use summary.gbm? Thank you!