You are spot on. If you only bid around 25% of the maximum then you will only show 25% of the time, spread evenly throughout the day.
It's usually because of your campaign doing well with a decent CTR that creates this situation, which is why it frequently happens.
On some of the accounts we manage we use that as the throttle t o give the clients more or less traffic according to their needs or their budgetary constraints. The good thing is that the CTR is consistent if you do decide to increase or decrease your daily spend limit.
One way to check if you are getting that happen is to check on the accounts page. If you see campaigns with lots of Overdelivery credits (free traffic), then chances are your campaign could be doing better than it is if you were prepared to pay more for traffic.
We check for this rarely on accounts where the maximum daily budget is seldom reached and daily for those accounts where it is. It's often the arenas where click spam is rife that it needs to be watched more closely.
In cases where this happens it's often one or two keywords that create the problem and pulling those keywords into campaigns on their own usually helps the other keywords get more exposure.