MXF OP1A XDCAM50HD 720p 4:2:2

I have been exporting :10-:15 sec spots using this setting and the bitrate noramlly passes, whcih needs to be 49.5-50.5 but for some reason when I export a :05 spot the bitrate comes in smaller 49.1 which get rejected automaticly. I can't find a bitrate setting with this option. From what I find XDCAM50HD should be exporting at 50mbps. Is that correct? Does anyone know how I can correct the bitrate for these smaller :05 spots?