The system will take the steps of the input device (manual entry or connected device) with the most steps and ignore all other entries from the same day. This is important as some people have multiple devices tracking their steps (sometimes event at the same time). To prevent double counting the same steps only one input method will be used for each day. It is perfectly okay that different days use different input methods.
In case the daily steps is incorrect for whatever reason, you can make a correction using manual entry. To make a correction you should make a manual entry with the total daily steps and not the delta.
Example: when you did 20,000 steps but your Apple Watch only registered 15,000, you should make a manual entry of 20,000 for that day to correct for this.