Half of 301 is 150.5 and half of 199 is 99.5. JavaScript Math.round produces 151 and 100.
Round each source axis once
The tool does not derive the second side from an already rounded first side in percentage mode.
Rounding remains visible
The result reports both axis scales and labels ratio preservation within pixel rounding.
Reproduce the result
Choose any ordinary image with the stated source dimensions, enter the example setting with aspect lock enabled, and generate the file. The displayed output dimensions must match this example; byte size can differ because pixels, format, quality and browser encoder also matter. Repeat the example with PNG and JPEG to see that identical dimensions do not imply identical bytes. Download each result and reopen it before relying on the browser preview. The source remains the reference if a receiving service alters the file again.