Could be any number of things, including the coffee beans.
Two different machines -- a commercial one and your Silvia---will produce different results, and these will be compounded by the two different barista techniques.
Are you temperature surfing your Silvia, or extracting at any old time? If the water is too hot, as it will be during part of the heating cycle, the coffee will emerge burned and bitter.
Are you dosing enough? Too little coffee and too hot a temperature will certainly be bitter.
Measure out about 17-18 grams of ground coffee, tamp it correctly and extract it in about 30" . Pull the shot 50" after the light goes ON.
There are many variables, and even with two Silvias side by side, using the same coffee, we would produce different results.