Help needed exporting large image sizes

Hi All,

I'm a newbie so forgive my lack of understanding here when it comes to exporting to large file sizes. In short I'm trying to export some images for large format printing but am having trouble preparing the right size in LR4.

Specifically I'm trying to produce w 30" x h 20" images. I understand that to do this I'll need a image that is 9000 x 6000 pixels set at 300 ppi. My problem is that when I export this, with the dimensions set to width and height in pixels and resize unchecked LR4 produces an image that is 9000 x 4887?

Would appreciate any thoughts on what i'm doing wrong here? Is it simply that the aspect ratio of image is wrong to produce this size image? My concern is not producing an exact file size for printing.

Thanks!

Tom