Transaction counts and times come from the latest 100 transactions, which reach back 4 blocks and cover 4 of these 16. A dash means the block falls outside that window — not that it is empty. Rialo has no way to ask for a block without its full transaction list, so covering the rest would cost one whole-block request per row — which has meant anything from a kilobyte to 91 KB and five seconds per block, depending on the node build. Open a block to read its transactions directly.