Reports originating from XDA forums suggest that the fresh units of Nexus 5 reaching consumers differ slightly from the older units. According to users, Google and LG seem to have made little tweaks to the buttons and speaker grill of the phone to make them better.
The power button and volume rocker on the newer Nexus 5 units is no longer loose (or at least appears to). Also, LG has made the holes in speaker and primary microphone grill slightly bigger, which might help in better sound output/ input.
You can check the serial number of your Nexus 5 box to see whether your phone belong to older stock or the newer one (newer units seems to have s/n starting 311), which can also be verified by simply looking at the speaker grill, the change in grill is quite visible.
The hardware tweaks are nothing new and manufacturers often make small tweak to the design of the devices after getting feedback from the initial buyers. Even the last Nexus smartphone – Nexus 4 had seen a similar design refresh.