Irish Breakfast teas are a blend, and while they tend to have a fairly consistent character (strong, malty usually), there are sometimes major differences from one to the next. Usually Irish Breakfast has mostly Assam or all Assam, but the other teas used in it can vary.
If you're used to an Earl Grey with a particular base tea, like an Assam, or Ceylon, and then you encounter an Irish Breakfast with a large portion of a similar tea, it's going to taste quite similar.
Rarely, there are even some pure black teas that naturally have citrusy tones much like an Earl Grey. I've found this less common with the types of teas that are typically put into Irish Breakfast, but it's certainly possible.
I also think it's great that you noticed how the character of the tea changes with each infusion. This can be fun and interesting, allowing you to experience the tea in different ways. Although black teas and British- or Irish-style teas are less commonly brewed in this way, if you find a tea that works well like this, then that is great, you can get more mileage from the leaf as well as experiencing it more richly.