Why does joining 5 threads at once result in simultaneous activity?

Thread.new{sleep rand(0..10}; puts '#1 done'}.join
Thread.new{sleep rand(0..10}; puts '#2 done'}.join
Thread.new{sleep rand(0..10}; puts '#3 done'}.join
Thread.new{sleep rand(0..10}; puts '#4 done'}.join
Thread.new{sleep rand(0..10}; puts '#5 done'}.join

This will result in thread 1 being executed to completion before thread 2 begins. I will always get the output 1-2-3-4-5 and the batch can take up to 50 seconds.

t1 = Thread.new {sleep rand(0..10); puts 'Thread 1 done.'}
t2 = Thread.new {sleep rand(0..10); puts 'Thread 2 done.'}
t3 = Thread.new {sleep rand(0..10); puts 'Thread 3 done.'}
t4 = Thread.new {sleep rand(0..10); puts 'Thread 4 done.'}
t5 = Thread.new {sleep rand(0..10); puts 'Thread 5 done.'}
[t1,t2,t3,t4,t5].map(&:join)

This, however, results in all 5 threads being executed in parallel; each time I will get a random output order and the entire batch takes no more than 10 seconds.

My question is: Why?

My understanding is that map will go through each item in the array and execute the Thread#join method on each one in turn... which is exactly what my first code example does. So I would expect totally identical results, yet there's clearly some difference there.