This is not a bug. This is not a feature too. This is just an interesting search result that you want to try out with your Google Home or Google Assistant on your phone.
According to Android Police:
Unfortunately, Google Home is unable to translate this into 6.25lb, and instead spits it out as 6.250000000000001lb. Interestingly enough, any conversion from 4oz to 4lb 4oz is fine, but after 5lb 4oz, things go haywire. There are other cases of this, too; 6lb 12oz reveals a similar issue.
Here is a video of this in action:
Try it out!