This is a common issue. The software manager starts searching for what ever you've typed into the search box as soon as you've typed it.
And it searches EVERYWHERE in the repositories list, including summaries.
kc1di has a fix that might work in this post in another thread on the same issue: viewtopic.php?f=90&t=120274#p684929
For convenience, here's what to do:
kc1di wrote:go to Software manager and then edit tab and uncheck the search in package summery...
Synaptic's better at the job though, although it's not as easy to get into at first.