When is a swamp not a swamp?

This may not be a problem, but something does not feel right about it. When my Cyclopean Giant was put into the graveyard it targeted the AI Razorverge Thicket. The giant got exiled, the thicket says it is a Swamp and can now be tapped for G, W or B mana. Then in the same game I played Evil Presence on AI Plains and it is a Swamp but can only be tapped for B. Are both of these correct? One says "becomes a Swamp" the other says "is a Swamp". Is there a difference between Land and Basic Land?