There are times that some members changed this, if it is testimony Sunday they talk like a speaker, but if we call them as a speaker they didn’t accept it. What is the difference between a testimony and speaker?
From my point of view I don’t think there is a difference between a speaker and a testimony (a testimony is given by a speaker). A testimony is supposed to be a short declaration of what we know or believe to be true as received from the Spirit of the Lord. A talk is focused on a theme/topic by which a member then will share their thoughts according to the things they have studied.
A testimony doesn’t require a person to be studied on a particularly topic. Some members are more comfortable with sharing their thoughts and feelings (which as you say is more like a talk rather than a testimony) because they don’t have to prepare anything. Some members feel uncomfortable because they feel like they have to be a scriptorian in order to give a talk. In that case, they will more likely reject the opportunity to speak — give a talk — because they feel unqualified.
If you know a person struggles with accepting a talk, then ask them to give a testimony on the subject/topic for the month. They may be more likely to accept the invitation to share their testimony on a topic rather than feeling like they are unqualified to speak on a topic. Sadly, this is a stigma in our wards — our culture — that people feel they have to be so thoroughly studied in order to give a talk.