This a normal behavior considering how Muse works and how different sites on Internet respond to consecutive searches.
A search performed in a Muse application starts at the ICE server level a number of connectors, each working on a native data service. Each such connector, as it searches the native data service and obtains records, sends these records to ICE which further sends records to the Muse application to be displayed.
So, the records come and are displayed in the Muse application www interface in the order in which they are received from the native sites and parsed. From one search to another the native sites might have different speeds of response and also the order in which the connectors are launched and run into execution may differ because they are run in parallel as threads.
Considering this, it is possible that at a second search performed in a Muse application, using the same term and on the same sources, to show records in different order.