Yes, the time frame is the same, but the meaning is different: this is because the aspect is different. As well as tenses (past/present), English verb forms have aspects: continuous and perfective. In general terms, the continuous aspect focuses on an action in progress, while the perfective aspect focuses on the achievement or result of the action. Here are two examples of contexts: 1. "What's all that mess outside the neighbours' house? There are bottles and broken glasses everywhere!" "They must have had a party". Here, we are focusing on the party that obviously took place and the result of that party in the present. 2. "What was all that noise going on last night?" "It was from the neighbours' house. They must have been having a party." Here, the focus is on the party as it was happening and the noise it was creating at the time. Can you see the difference?