Understanding why baselines use two intervals
Using two reporting intervals is a compromise between having enough data for a useful comparison and the amount of data stored on disk.
Apex uses data at the same granularity to create a baseline comparison. The granularity is set by the dashboard time setting. That is, if you are looking at an Hour of data, it is compared to another Hour of data. This is normally fine if the data you are looking at is relatively recent. It becomes a data storage challenge to compare an hour from yesterday to an hour from five years ago.
Because Apex does not have unlimited storage capacity, there is a trade-off between data retention and how far back in time to allow comparisons for smaller reporting intervals.
For intervals of Day or less, there is enough storage capacity to calculate a baseline from the same day up to two weeks ago. For intervals longer than a day, the preceding two intervals of the same length are used to provide a valuable baseline.
For this reason the number of intervals is fixed at two and cannot be modified. Two intervals offers a balance between enough data for a useful comparison while managing the data storage requirements.