In general

X is deceptively Y

can be interpreted as

X has a greater quality of Y than you might at first assume or observe

Now, with regard to

He looks deceptively strong

Someone with experience of this deceptiveness, let's say for example a pro-wrestler, might observe some other wrestler, a thin one, and conclude, based on their experience, that the skinny appearance belies a strength or compensating quality that should not be dismissed or underestimated in the ring. To this pro-wrestler the skinny person appears strong, precisely because to the average non-wrestler they would appear weak.

To the observing wrestler, the skinny wrestler appears both strong (to her) and deceptive (to others), hence looking deceptively strong.

In general

X is deceptively Y

can be interpreted as

X has a greater quality of Y than you might at first assume or observe

Now, with regard to

He looks deceptively strong

Someone with experience of this deceptiveness, let's say for example a pro-wrestler, might observe some other wrestler, a thin one, and conclude, based on their experience, that the skinny appearance belies a strength or compensating quality that should not be dismissed or underestimated in the ring. To this pro-wrestler the skinny person appears strong, precisely because to the average non-wrestler they would appear weak.

To the observing wrestler, the skinny wrestler appears both strong and deceptive, hence looking deceptively strong.

In general

X is deceptively Y

can be interpreted as

X has a greater quality of Y than you might at first assume or observe

Now, with regard to

He looks deceptively strong

Someone with experience of this deceptiveness, let's say for example a pro-wrestler, might observe some other wrestler, a thin one, and conclude, based on their experience, that the skinny appearance belies a strength or compensating quality that should not be dismissed or underestimated in the ring. To this pro-wrestler the skinny person appears strong, precisely because to the average non-wrestler they would appear weak.

To the observing wrestler, the skinny wrestler appears both strong and deceptive, hence looking deceptively strong.

In general

X is deceptively Y

can be interpreted as

X has a greater quality of Y than you might at first assume or observe

Now, with regard to

He looks deceptively strong

Someone with experience of this deceptiveness, let's say for example a pro-wrestler, might observe some other wrestler, a thin one, and conclude, based on their experience, that the skinny appearance belies a strength or compensating quality that should not be dismissed or underestimated in the ring. To this pro-wrestler the skinny person appears strong, precisely because to the average non-wrestler they would appear weak.

To the observing wrestler, the skinny wrestler appears both strong (to her) and deceptive (to others), hence looking deceptively strong.