The likely reason for the difference between online ocr form result from on the front page and your API call is that the online form uses the parameter scale=true in its API call.
So if you use the OCR API, you need to set the scale parameter to true in the call. When the parameter is missing, the default is scale=false. Once you do that, the results should be identical.
scale [Optional] true/false
If set to true, the api does some internal upscaling. This can improve the OCR result significantly, especially for low-resolution PDF scans. In the front page demo, this parameter is set to true.