Why does my Apple health not reflected in My Fitness?

It matters what you're looking at here. Can you be more specific?

On the Apple side, I'm assuming you're either looking at the calorie total from an exercise-session-type walk, or your active calories either for the day or during the walk time period, or something like that?

What are you comparing it to on the MFP side: Calories specifically labeled as being for that exercise session? The calorie adjustment in your eating calorie goal? Something else?

The number of calories added to your eating calorie goal will almost always be different from the number of calories from an exercise session, different from active calories on the Apple side, etc.

This is oversimplified, but the basic reason is that your MFP calorie goal is based on your activity level setting in your MFP profile, and even the lowest activity setting (sedentary/not very active) assumes some amount of walking or other calorie-burning activity from daily life movement. How many calories it assumes depends on which activity level setting you choose.

For any fitness tracker synced to MFP, by the end of the day the adjustment in your eating calorie goal would be the difference between how many calories MFP expected you to burn in all ways based on the totality of your profile setttings, vs. the total number of calories your device estimated you burned based on all the movement patterns it detected, including intentional exercise. Only wildly by accident would that adjustment number exactly match the calories from an intentional exercise session.

But if that's not the calories you're looking at on the MFP side, we'd need you to be more specific. Even a screen shot of the calorie reconciliation on the MFP side would be ideal.

Best wishes!