Thanks for reaching out to us! This is done by default so as not to distort your image, because if you resize to a different proportion than the original your image will be stretched or squashed. However, you can override this by unchecking the Keep Proportions checkbox within our Resize tool.
The other thing you can do is to first Resize and keep the proportions, and to then Crop in order to get the exact dimensions you want.
Let us know if you have any other questions!
Regards – The Ribbet Team