When I use the Java reflection API to get information about all methods and fields of the interface clojure.lang.IPersistentVector, e.g. using code in the REPL like this:

(require '[clojure.reflect :as refl])
(require '[clojure.pprint :as pp])
(def t1 (refl/type-reflect clojure.lang.IPersistentVector))
(pp/pprint t1)

I see not only the methods that are explicitly declared in that interface here: https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/IPersistentVector.java

I also see a fourth method named cons that has the flags :public :bridge :synthetic that has :return-type clojure.lang.IPersistentCollection. My guess is that this is somehow because IPersistentVector extends IPersistentStack, which in turn extends IPersistentCollection, which has its own cons method.

The Java Virtual Machine Specification for Java SE 8 says that the bridge flag for a method means "A bridge method, generated by the compiler.", and the synthetic flag means "Declared synthetic; not present in the source code." (see https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.6).

Hmmm, I was going to say I had not found much in the way of explanation for why this happens, but this may be a case of rubber ducking, as I am perhaps doing different Google searches now than I was earlier when I tried to learn what is going on. This article might be explaining what is going on: http://stas-blogspot.blogspot.com/2010/03/java-bridge-methods-explained.html#:~:text=Bridge%20methods%20in%20Java%20are,the%20actual%20method%20being%20invoked.