When you actually read the documentation for exist, you will find that an output of seven means that exist has located a folder with that names, not a variable as you think it has. So somewhere on your path there are folders with those names. And running clear makes no difference to folders. If you want to only search for variables, and not all of the other useful things that exist can locate (variables, mex files, simulink models, folders, functions, etc), then you should specify the optional argument like this and specify the output value:
X = 1==exist(name,'var');
really you should do this every time you use exist.
And of course the documentation has lots of other useful information on how to use MATLAB functions properly, so perhaps you should take a look!
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