IBM interview process is inconsistent - some departments are very thorough, others ad-hoc. Departments that are doing a lot of hiring tend to have a well defined process, while those hiring for an occasional backfill may not be that good at assessing your skill, or even following up when they have reached a decision. You could find yourself interviewing with one manager only, or a manager and a developer (separately), or up to 5-6 interviews. Remember, the more interviews, the more thoroughly you are being assessed, which means an offer will be a good match. If you get an offer after just one interview, don't expect it to be the best job.