One sequence is partly contained in the other (highlighted).
So the CSA sequence is (FASTA format, truncated):
>sp|P29474|NOS3_HUMAN Nitric oxide synthase, endothelial OS=Homo sapiens GN=NOS3 PE=1 SV=3
MGNLKSVAQEPGPPCGLGLGLGLGLCGKQGPATPAPEPSRAPASLLPPAPEHSPPSSPLT
QPPEGPKFPRVKNWEVGSITYDTLSAQAQQDGPCTPRRCLGSLVFPRKLQGRPSPGPPAP
EQLLSQARDFINQYYSSIKRSGSQAHEQRLQEVEAEVAATGTYQLRESELVFGAKQAWRN...
taken from http://www.uniprot.org/uniprot/P29474 for convenience.
While the PDB one is :
>3NOS:A|PDBID|CHAIN|SEQUENCE
PKFPRVKNWEVGSITYDTLSAQAQQDGPCTPRRCLGSLVFPRKLQGRPSPGPPAPEQLLSQARDFINQYYSSIKRSGSQA
HEQRLQEVEAEVAATGTYQLRESELVFGAKQAWRNAPRCVGRIQWGKLQVFDARDCRSAQEMFTYICNHIKYATNRGNLR
SAITVFPQRCPGRGDFRIWNSQLVRYAGYRQQDGSVRGDPANVEITELCIQHGWTPGNGRFDVLPLLLQAPDEPPELFLL...
The Uniprot entry mentions 3 different isoforms due to alternative splicing, so perhaps that is what is going on here. Here is the output from a sequence alignment (using https://www.ebi.ac.uk/Tools/psa/emboss_matcher/):
#=======================================
#
# Aligned_sequences: 2
# 1: NOS3_HUMAN
# 2: SEQUENCE
# Matrix: EBLOSUM62
# Gap_penalty: 14\
# Extend_penalty: 4
#
# Length: 240
# Identity: 240/240 (100.0%)
# Similarity: 240/240 (100.0%)
# Gaps: 0/240 ( 0.0%)
# Score: 1294
#
#
#=======================================
NOS3_HUMAN 66 PKFPRVKNWEVGSITYDTLSAQAQQDGPCTPRRCLGSLVFPRKLQGRPSP 115
||||||||||||||||||||||||||||||||||||||||||||||||||
SEQUENCE 1 PKFPRVKNWEVGSITYDTLSAQAQQDGPCTPRRCLGSLVFPRKLQGRPSP 50
NOS3_HUMAN 116 GPPAPEQLLSQARDFINQYYSSIKRSGSQAHEQRLQEVEAEVAATGTYQL 165
||||||||||||||||||||||||||||||||||||||||||||||||||
SEQUENCE 51 GPPAPEQLLSQARDFINQYYSSIKRSGSQAHEQRLQEVEAEVAATGTYQL 100
NOS3_HUMAN 166 RESELVFGAKQAWRNAPRCVGRIQWGKLQVFDARDCRSAQEMFTYICNHI 215
||||||||||||||||||||||||||||||||||||||||||||||||||
SEQUENCE 101 RESELVFGAKQAWRNAPRCVGRIQWGKLQVFDARDCRSAQEMFTYICNHI 150
NOS3_HUMAN 216 KYATNRGNLRSAITVFPQRCPGRGDFRIWNSQLVRYAGYRQQDGSVRGDP 265
||||||||||||||||||||||||||||||||||||||||||||||||||
SEQUENCE 151 KYATNRGNLRSAITVFPQRCPGRGDFRIWNSQLVRYAGYRQQDGSVRGDP 200
NOS3_HUMAN 266 ANVEITELCIQHGWTPGNGRFDVLPLLLQAPDEPPELFLL 305
||||||||||||||||||||||||||||||||||||||||
SEQUENCE 201 ANVEITELCIQHGWTPGNGRFDVLPLLLQAPDEPPELFLL 240