when you look at laptops and it gives you a maximum screen resolution of 1600x900, that is the amount of pixels the screen has and is it's natural resolution it's not really a determination of what resolutions the laptop is capable of displaying though.
the graphics inside is capable of displaying a much lower or higher screen resolution if required, so if you connected a 1920x1080 monitor then you could change the resolution from 1600x900 to 1920x1080 and have a natural or pixel to pixel screen resolution displayed on the monitor.
as sleepydude said there are 2 factors that need to be taken into account when talking about resolutions first is a screens natural resolution and second is what the graphics device is capable of outputting, but seeing how you're looking at laptops see if the below link helps you decide.
No single display specification automatically makes it better than another. It’s the combination of specifications that makes the display what it is.
Edited by terry1966, 24 August 2017 - 08:12 PM.