Hi
When recording a workout via the Workout app, choose the activity type that most closely matches your workout. This enables Apple Watch to use the most appropriate sensors and data sources when estimating the results, including using tailored calorie calculations for preset activity types.
For Outdoor Walk workouts, data sources include Location Services / GPS data when it is available. (Apple Watch Series 2 has built-in GPS capability, whereas other models can use GPS data from the paired iPhone, if it is also taken along).
In the absence of GPS data, the watch will rely your arm movements, as measured by the built-in accelerometer, when estimating results including pace and distance. When mowing a lawn, your arm will not swing in the same way as it would when walking, so estimations made on this basis may be inaccurate.
For activities that are not included within the list or that are not sufficiently similar, choose to record your workout under "Other". This credits one minute of Exercise for each full minute of the workout. Active calories for Other workouts are estimated at a rate equivalent to a brisk walk or based on data recorded by the heart rate sensor, whichever is higher.
These factors likely explain the difference in the results that were recorded for your workouts.
More information:
Work out with your Apple Watch - Apple Support