Keep in mind IR thermometers are sensitive to the reflectivity/emissivity of the materials they are used on, and the shiny PEI surface can be a bit tricky. Try measuring the temperature of a mirror that your warmed up with your hand and you’ll see a similar problem.

If you want to test more accurately, put some matte tape on the bed and then measure the temperature then.

That said there is also a chance for the heat bed to be a bit lower than target temperature, since the temperature control and feedback occurs at the bottom of the bed where the thermistor is, not at the top, and is separated by a layer of magnetic adhesive and coated metal.

I wouldn’t worry about the value difference too much but more the consistency. If you need 75 degrees on the edge then keep dialing up the temp until you get it.

It’s most likely only going to affect large prints at the end of the day.

Your printer will measure the bed from a single location so moving your IR sensor will vary across the bed. If your using a custom firmware I’m sure you could set this as an offset assuming your IR thermometer is accurate ;)

Heat of the bed will be the warmest point and the periphery cooler (just like your body temp)

Adhesion I’d totally recommend cleaning your surface with soap and water (surface depending) and then messing with temps. With abs and petg I’ve found that even a bit of finger print residue is enough to cause a part to come away from the bed, next I’d check first level height. I’ve very rarely found the bed temp to be the reason I lost adhesion.

What the heck are you printing that requires 75C of bed temp with no walls?