It's just because of the way the card is written (literally, you HAVE to look at the fine print):
Stumbling
Continuous Spell
As long as this card remains face-up on the field, any monster that is Normal Summoned, Flip Summoned or Special Summoned successfully is changed to Defense Position.
Level Limit - Area B
Continuous Spell
All face-up Level 4 or higher monsters on the field are changed to Defense Position and remain in Defense Position as long as this card is active.
The text on "
Stumbling" is in response to a Successful Summon so it has a "timing issue" as to when it activates (despite being a Continuous Spell... that just means the card stays on the field but isn't always active). "
Level Limit - Area B" doesn't have that qualification.
The only card I can think of to differentiate the 2 is "
Big Shield Gardna". If he's attacked while in defense position, face-down, his effect will put him into attack position afterwards. Now if "
Stumbling" were on the field, nothing would happen since he wasn't Summoned in any fashion. But if "
Level Limit - Area B" were on the field, that effect would put him back into defense position immediately afterwards.