My husband and can walk together with our phones. His iPhone app records more steps/miles than mine. Why?

If either of you are wearing a paired Apple Watch, that will be used to count steps and estimate distance. See Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support

If it’s purely iPhone‘s you’re both using, there is some discussion here about step count accuracy.

The height entered under Settings > Health > Medical ID may vary the step count to distance ratio. Weight might also be used as a factor. It might be worth checking those are correct.

See also Health App Step Length - Apple Community

Hello moving2move,

Welcome to Apple Support Communities! I understand your husband's iPhone record more steps/miles than yours when you cover the same distance. I'm happy to assist you with this.

The Health app gathers data from different sources such as an Apple Watch and other apps that are compatible with the Health app. This could be the cause. You can read more about it here: Use the Health app on your iPhone or iPod touch

If you feel your data is incorrect in the Health app on your iPhone, close your apps and restart the iPhone to see if anything changes:

Restart your iPhone, iPad, or iPod touch 

Best Regards.