During our recent holiday we took a surfing course. The instructor agreed to pick us up the first day at 9am. Since we were in the Caribbean I assumed he would show up around 9:15am, but to my surprise he came at 9am sharp. The next day he came at 9am again. I figured it was his German origins and concluded he would be on time in the future. So when he hadn't showed up at 9:05am on the third day I got annoyed. He eventually arrived around 9:07am and was very apologetic for being so late. Truth is, seven minutes is nothing in the Caribbean - particularly for a surfing lesson! But by being on time the first two days Marcus had set up an expectation in us that he would be on time in the future? It reminded me of something that happened in the office a month ago: we have a new contractor to whom we agreed to pay Net-30, i.e., within 30 days of the end of the month. During the first week of the month one of my co-workers asked me to pay the contractor for the previous month. I said no. I said I didn't want to set the expectation in him we would pay him on the first week of the month. I suggested waiting until the 2nd or 3rd week of the month.
Sometimes it makes sense to "under-deliver" to set "appropriate" expectations and then be able to exceed them in the future.