The initial load balancing decision in Round Robin mode should be uniform, but several factors can influence whether the traffic continues to appear uniform in the stats, the primary one being persistence. If you are using source IP persistence on the virtual server that uses this pool, any clients that come from behind a NAT or proxy will appear as a single client to LTM and will be persisted to the same server. This could lead to a disparity in the number of connections to each pool member. Cookie persistence can alleviate this somewhat.
Secondly, if OneConnect is enabled then LTM attempts to re-use server side connections so as to reduce TCP overhead between LTM and the servers. So multiple connections on the front side end up as one connection on the back end.
Thirdly, it could just be random traffic patterns, if the users on one server happen to access more content than users on the other server, their request count could end up higher just as random chance.
Denny