Start with the retained rectangle
A 4000 × 3000 source filling a 10 × 8 inch frame retains 3750 × 3000 pixels. Divide 3750 by 10 and 3000 by 8: both give 375 PPI. The matching values show that the retained region is placed with one uniform scale.
Using the full original width would produce 4000 ÷ 10 = 400, while the height still gives 375. Those different numbers do not describe a faithful fill placement. They reveal that you have mixed the original dimensions with a frame that has a different shape.
Compare the actual image size in fit mode
Keep the same photo whole inside the 10 × 8 frame and its image rectangle is 10 × 7.5 inches. Divide 4000 by 10 and 3000 by 7.5 to get 400 PPI. The remaining half inch of frame height is border, not photo.
Do not divide by the full eight-inch frame height in this fit calculation. Doing so would count the border as though it contained image pixels. For image PPI, the relevant physical rectangle is the photograph itself.
A crop does not always lower PPI
Consider a 6000 × 4000 source printed whole at 12 × 8 inches: it gives 500 PPI. Change to a 10 × 8 frame and fill it. The retained rectangle is 5000 × 4000 pixels, and it still gives 500 PPI. The crop removed width, while the physical width decreased proportionally.
Now enlarge that retained crop to 20 × 16 inches. Its PPI falls to 250 because the same pixels occupy twice the length in both directions. The cause is the relationship between retained pixels and physical size, not cropping considered in isolation.
Separate position from scale
Moving a largest-fit crop left or right selects different source content without changing its retained dimensions. Its effective PPI therefore stays the same. The crop percentage and position still matter because they describe what is lost and where.
A tighter crop would use fewer pixels for the same physical rectangle, reducing PPI. If another editor has already made that tighter crop, read the dimensions of that file rather than assuming the original camera dimensions still apply.
Interpret the number with the image
A PPI value is not a guarantee of visible quality. Sharpness, noise, compression, processing, paper, and the way the print is viewed all contribute. A source with more pixels can still contain less useful detail than expected.
Use the number to compare alternatives honestly: a smaller print, a less restrictive crop, or a different source. Check the exported raster’s pixel dimensions after rounding and use the paper-layout PDF when physical placement matters. Keep a measured print test as the final check when the result is important.