I thought you would know what dpi is, but terms get convoluted there fore can be missleading.
You can in the scanning in, HP Scan. The setting of resolution is under Image type and pick custom, That should give selections, the point where the list "enhanced" is the point where your actual scanner stops it's built in abilities. Like the "digital zoom on a camera opposed to optical - optical is "real" and digital is it's "interpretation."
One thing that may have happened too is...well. Let me start over, the ability to enlarge and reduce give the opposite results. You probably know this too. But taking a little pic and making it bigger will force a program to "interpret" what needs to be added or stretched to make the pic larger (this can give exactly what you were talking about earlier). When I worked for a printing company that we often did posters and "Artist Prints," we would scan at a larger size than we would be printing out - say 2-3% larger. and reduce to sharpen the image. Not always, but most of the time.
I probably said more then you asked, but I am just trying to be as helpful as possible. Maybe I should change my custom title to "TMI."