Numerous people cannot remember the names of acquaintances they've lived and worked with for years. If you name a concept a programmer works with every day, it's entirely reasonable that that programmer does not remember the name because she knows the concept by its syntax, not its name.

A lot of interview questions are also asked using synthetic examples that are not complete solutions. A good programmer will reach out and explore the full technical ramifications of the requirements instead of merely solving the problem. Knowing that complexity adds a whole new level of stress versus simply solving the easy problem the interviewer intended.

A lot of people have trouble sleeping the night before an interview. It's more stressful than anything they will likely encounter at your company because it determines a major, unpredictable branch point in their future. Unless informed their continued employment is at risk over the outcome of a meeting, client meetings will not have the same degree of stress.