Calories seem inaccurate (iPhone)

With the iPhone, calorie estimates are very much that - estimates. On the iPhone Slopes doesn't have access to a heart rate sensor to allow Slopes to account for how hard you rode (like with Apple Watch).

If you're recording on iPhone and find that your calorie count appears to be significantly inaccurate, it's worth checking that your weight has been entered correctly via the Account tab under Settings → Integrations → Your Weight.