You're seeing them being used to express the same idea because they can have the same meaning. They can both mean "In my opinion, ...". It's just that sometimes people want to appear more humble and less strict with others' opinions or performance that they use "I guess". The other difference is that "I think" sounds and is more certain. It's used when you're sure of what you're saying and to create a more persuasive tone. When you use "I guess", it can be purely a guess or it is because you don't want to sound too serious or strict. Example: I think Sue is dishonest. So you'd better not count on her. Compare: I guess Sue is not being that genuine. You may want to consider giving the job to others. From the above example, I hope you can see the difference between the tones.