access controller variables in a model

[eluser]Unknown[/eluser]
i dont know, i have been trying to find a logic in it for hours ... but i cant find out

from a model i can access a variable declare in the calling controller
i can use it, echo it, and display his content

but i can not do an isset on this variable
it will return false

to be able to test if a variable exists in my controller from my model, i have to use & get_instance(), and then i ll be able to test if the var exists or not

even if i display the defined variables in my model, it wont display my controller's variable, but still i ll be able to use those variables ???

can anyone explain me please

cheers