I just acquired a used 5500 lumen Optoma EH515 and when I look at the throw calculator it shows at 17ft and 150' screen an est image brightness of 60 fL and my existing 4500 Lumen InFocus IN3138HDa has a 70 fl.

Can someone explain why it shows a lower image brightness with higher lumen?

Looking at the calculators and using the manufacturer's specs, which are almost always WAY too high than what they can really put out with decent color accuracy, the Optoma is listed as 64fL vs. 61fL for the same 150" diagonal screen size.

https://www.projectorcentral.com/Optoma-EH515-projection-calculator-pro.htm

https://www.projectorcentral.com/InFocus-IN3138HDa-projection-calculator-pro.htm

Be aware that Projector Central may default to their 'estimated brightness' which is about three quarters of half of what the manufacturer claims, and is often much more accurate of a expectation for real world use.

I would move it closer, as you intend to, so you can maximize light output if you are fighting any ambient light in the room, this is a good course of action.

Light output levels have very little to do with color saturation. Color saturation is determined by the design of the projector and the color wheel that is in use. You can have incredibly bright projectors with very accurate colors, but these cheaper bright models tend to use white light to boost their claimed brightness and tend to have rather muddy colors when at full brightness. So, peak brightness claimed is not a usable condition for watching video content, but it is often fine for PowerPoint presentations and casual viewing.

In a completely dark room, you may find the brightness to be more than you can comfortably enjoy. But then you can use low lamp mode and once calibrated for best color accuracy you may find it delivering well under 2,000 lumens.