Depending on the settings of Settings / Preferences and Project Options the duration seen in the Slide List can change.
If a single slide is added to the Slide List the Main Screen View looks like this:
The Status bar shows the Full (Slide) Duration and the (Slide) Duration. The Slide Icon displays the Slide Duration. For a single slide the Duration will equal the Full Duration because there is no following slide transition time to take into account - or the following slide transition time is zero.
The corresponding Objects and Animation Screen looks like this:
Adding a second slide with a 2 second transition time causes a change in the displayed (Slide) Duration. Note that the Full (Slide) Duration remains the same.
Objects and Animation View:
The (Slide) Duration of Slide 1 remains at 5 seconds but because Slide 1 is visible from the beginning of its own transition to the end of the Slide 2 transition (at varying opacities) its Full (Slide) Duration is 7 seconds.
Adding a second slide with a 2 second transition time causes a change in the Duration Icon on the Slide. Note that the Full (Slide) Duration and Full (Slide) Duration remain the same.
The Objects and Animation Screen remains the same.