Thanks for the report. As usual, you're right, and I've confirmed the gap.
For context on what each designer does today: in the desktop designer, dragging a control shows a live on-canvas measurement (X/Y while moving, size while resizing - the ruler button in the designer toolbar toggles it), and resizing also updates the Inspector fields in realtime. In the Web designer, the control preview and the dashed "landing" outline follow the pointer while you drag, but the actual Row, Column, and Span values only appear in the Inspector after you release the mouse button.
I think this matters more in the Web designer than on desktop, since placement there is grid-based: with a fixed number of columns and controls auto-stacking, it's hard to tell exactly which cell and span a control will occupy mid-drag - particularly when another control pushes it to a different row than the one it visually appears over.
I've logged this as a feature: a live readout of Row, Column, and Span shown alongside the control while dragging and resizing, reflecting the placement the control will actually land in.
https://feedback.objo.dev/feature/1558