PDF printing

My boss created a PDF from an InDesign CS2 document and asked me to print it. The PDF opened fine on my machine. I looked it over, and sent it to the wide-roll printer, my HP5500PS 60".

I should give you some background: my boss loves gradients and drop shadows.

The resulting printout had white boxes wherever a drop shadow lay over a pantone spot color, as if the printer couldn't make sense of the layover. I did a quick google search suggested that drop shadows over spot colors in InDesign wasn't a good idea (nor are they a good idea visually) and that i should try converting (in acrobat) the colors to process. Didn't work. I also visited the macDesign forum

Instead a ventured into my boss' office and had him show me how he exported the file: exported to PDF from InDesign CS2 using the default settings (using acrobat 5). For shits and giggles i had him export a new version using Acrobat 7. This not only cleared it up, but it also rendered his color more reliably, giving us more neutral grays and a truer color to our calibrated screens (mine, at least). Question left: Why would the resulting export using acrobat 5 look fine on screen (even after pre-flighting and looking at the overprint preview) and not print the same way?