This version is in the ratio of 5:4. This ratio is one of many "standard" ratios and is the basis for print sizes such as 8x10 and 16x20. Unfortunately, SLR cameras create photos in the ratio of 3:2 - thus the standard "snapshot" print size of 4x6.
For this 16x20 poster, using individual 3:2 ratio photos of the tree, there is an unavoidable white margin above & below each individual photo. If you look at the next version (click "next") you'll see that the individual photos fit within the four smaller frames without any excess white margin. This is because the overall poster size (13.33333 x 20) matches the size of the individual photos.
This issue could be addressed by suitable cropping of the individual photos. Unfortunately, that is not an option with these four specific photos (or any others of this tree) because there is no room to crop anything from the sides of each individual photo without ruining it.