By default, IO Ninja merges both RX (green) and TX (blue) data blocks to produce a more easy-to-follow log (the white-and-gray checker pattern background shows block boundaries). The timestamp shown on the left is the timestamp of the last block in this particular line.

You can also turn off merging altogether (Settings -> Log Engine -> Binary Data -> Merge -> Never) -- then each received block will be shown on a separate line, and you will see the timestamp of each block on the left.