Buses [Day 2]

Woah!!! We saw six buses today on our way to preschool. You might want to check out the Buses [Day 1] post from two days ago to understand the context of what's ahead. I was a little late to the bus-counting action on our way to preschool today so here's our first exchange.
Me: Have you seen any buses today?
Son (who turns 3 in a month): Yes. One.
Me: Was it a city or school bus?
Son: City. 
Me: Okay, we'll have to look for more buses today.

We immediately saw another city bus, bringing our total buses to two on the day. A little bit down the road was another city bus heading in the oposite direction.
Me: I see something on the other side of the road coming this way. What is it?
Son: It's another bus!! A city bus!
Me: How many total buses have we seen today? 
Son: Three.
Me: Good.
We continue along and a shorter city bus passes us. I don't point it out, but he spots it.
Son: There's another bus. 
Me: You're right. Wow! Now, how many buses have we seen?
Son: Hmph.
I pause and wait for him to process the question. Keep in mind, this is happening while I'm driving and he's in the backseat so I can't turn around to talk with him.
Son: Hmph
If I get a second "hmph" I know he doesn't have an answer and won't come up with one. Trust me, I've waited for long periods of time and will just continue to hear his cute little "hmph."
Me: Well, we already saw three buses and now we saw one more. What number comes after three?
Son: (whispering to self: one, two, three) Four.
Me: Good. So how many buses have we seen?
Son: Four.
Surprisingly, we hadn't seen a school bus yet. We were at our last red light and there it was in all its glorious mustardy-yellowy paint, a school bus. He exclaims, "A school bus!" Since we're at a red light, I turn around and ask how many buses have we seen. I get the two "hmph" count. I hold up one hand with four fingers up and the other hand with one finger up.
Me: We saw four city buses and now one school bus. How many buses have we seen today?
Son: Three
Did we just have a flashback (regression) to Tuesday?
Me: We did see three buses today, you're right. But, I think we've seen more. Count the fingers.
Son: One,... Two,... Three,... Five.
The light turns green and I have to go. I'm curious about him skipping 'four' and still landing on the correct number of buses. Seriously, what's up with that? I mumble to myself, "That's odd that he skipped four." Right as we're about to pull into the preschool parking lot, another school bus goes whizzing by in the opposite direction. He exclaims, "Another school bus!" Wow! We saw six school buses today, so I'm thinking we park the car and quickly review this last school bus.
Me: Wow! We just saw another school bus. We saw a lot of buses today. We saw five buses and now we saw one more. How many total buses did we see today?
Son: Hmph. (x2)
I hold up one hand with all five fingers up and the other hand with one finger up.
Me: So we saw five buses and we just saw another bus. Count the fingers.
Son: One,... Two,... Three,... Four,... Six.
Me: You're right. What happened to five?
He giggles! I do too because it's contagious. Seriously, what's up with this? He answered the correct number, but skipped the number directly preceding it. TWICE! He's happy he saw so many buses today. I am too. He's content with landing on the correct number. I'm perplexed.

Hmph,
1140

BTW. Thanks Christopher Danielson, for continuing to inspire me to have (and cherish) these conversations with my son.