B737-700 Captain reported receiving a 'TOO LOW' altitude alert from ATC. The Captain then realized the wrong altitude was set in the FMC. The Captain states CRM should have caught the error.

Narrative

Approach into ZZZ - Landing XX. After reviewing company documentation; we decided to brief and request the RNAV XX Approach even though it was day VMC. ATC offered the visual approach. We thought the best way to manage risk and plan the descent was to fly the RNAV approach in visual conditions. We were cleared direct ZZZZZ. Then cleared to cross ZZZZZ at 13000 ft. and cleared for the approach. As the Pilot Flying (PF); I set 5700 ft. in the ALT window and verified and called LNAV/VNAV PATH. When checking the FMC; I was sure that I was looking at 13000 ft. because that is what I expected to see. However; I realized later that I actually saw 11300 ft. for the segment between ZZZZZ and ZZZZZ1. ATC called when we went below 13000 ft. just prior to ZZZZZ with an Altitude Alert. We climbed back up to 13000 ft. and continued the approach. We were above the path at that point. We always had the terrain in sight and did not receive a GPWS caution. We continued the RNAV approach in visual conditions and landed without incident.First of all; this could have been prevented by more deliberate CRM. The First Officer (F/O) and I both thought we saw 13000 ft. in the FMC. 11300 ft. is similar. However; it should have been caught with CRM. In addition; it is rare to change or program an altitude on an RNAV (RNP) Approach that was loaded from the FMC. Besides the similarity of the numbers; it more common to get an RNAV approach clearance in which the first crossing altitude is already codified in the FMC. It is the norm. I should have caught this error. However; those two factors (different than the norm and similar numbers) lead to the expectation bias. Beginning descent below the ATC assigned altitude at the IAF on RNAV approach.